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Homology between DNAs of some orthopoxviruses: vaccinia, neurovaccinia, cowpox, ectromelia, variola and white-pox was studied. By the method of molecular hybridization of labelled virion DNAs of the viruses studied with an excess of unlabelled vaccinia DNA and further treatment of the hybridized products by the single-stranded-specific nuclease S1, it was shown that the concentration of unlabelled vaccinia virus DNA, converting 50% of homologous labelled DNA into a hybrid, converted under similar experimental conditions into a hybrid 50% of labelled neurovaccinia virus DNA, 48% of white-pox virus DNA, 43% of variola virus DNA, about 30% of cowpox virus DNA and 27% of ectromelia virus DNA. Based on the quantitative data obtained the relationship between viruses of this subgroup and the possible origin of vaccinia virus is discussed.

In 1980, the World Health Assembly declared the global eradication of smallpox and recommended the universal discontinuation of smallpox vaccination; nevertheless, it recommended that surveillance and research on orthopoxvirus infections should continue. By early 1982, all except 8 countries in the world had stopped routine vaccination programmes and all except 1 no longer required an international certificate of smallpox vaccination for travellers. Since 1978, as a result of continuing active surveillance, 176 smallpox rumours have been investigated in 60 countries. Two of these concerned the two laboratory-associated cases that occurred in the United Kingdom in 1978; all the others were false alarms. Special surveillance programmes for human monkeypox have been developed in West and Central Africa. The number of laboratories retaining variola virus stocks has been reduced to four. Investigations to determine the identity and origin of the six known isolates of "whitepox" virus have continued. Research on mapping of variola DNA and on monoclonal antibodies against certain orthopoxvirus antigens is continuing. All these measures are aimed at ensuring that the achievement of smallpox eradication is permanent.

Orthopoxvirus DNA from representative strains of rabbitpox, vaccinia, monkeypox, variola, cowpox and ectromelia viruses was analysed by cleavage with restriction endonucleases HindIII, XhoI or SmaI. Genome mol. wt. vary from about 120 x 10(6) for rabbitpox to about 145 x 10(6) for cowpox. Physical maps of cleavage sites are similar and characteristic for strains of the same Orthopoxvirus type. The distribution of HindIII sites suggests that an internal region of mol. wt. about 30 x 10(6) is highly conserved between Orthopoxvirus genomes although some type-specific differences occur within this region, especially with strains of ectromelia virus. Conservation of internal sequences is less marked following analysis with XhoI although cleavages within this central region of particular genomes appear to represent a subset of preferred sites. Endonuclease SmaI cleaves exceptionally infrequently and distinguishes variola, monkeypox, vaccinia, cowpox or ectromelia viruses. Type specific differences result largely from extensive, near terminal variations in length and sequence. Representative Orthopoxvirus genomes have rapidly renaturing terminal restriction fragments confirming the presence of near terminal, covalent cross-links. Terminal restriction fragments from the same or different genomes generally cross hybridize indicating the presence of near terminal repetitions of mol. wt. up to 6 x 10(6) and which share at least a subset of common sequences. Variola strains however, appear to lack such sequences from one specific terminus which maps shorter than that of related viruses.

Two variola-like viruses were isolated in Bilthoven in 1964 from monkey kidney tissue cultures. These viruses, coded 64/7255 and 64/7275, have been considered as two of the six "whitepox" viruses isolated from animal tissues, all of which are indistinguishable from variola virus by laboratory tests.Two specimens from suspect smallpox cases in India were examined in the Bilthoven laboratory at about the same time as the "whitepox" viruses and two strains of variola virus were isolated from them. A detailed comparison of certain biological markers of these four viruses, and of their DNAs, shows that the two "whitepox" viruses could not be distinguished from each other or from one of the two variola isolates. In view of this, and since there was a possibility of cross-contamination at the time of isolation, it is concluded that 64/7255 and 64/7275 must be regarded as genuine variola viruses and be deleted from the list of variola-like viruses isolated from animal tissues.

Structural polypeptides from a number of poxviruses were analysed by SDS-polyacrylamide gel electrophoresis. The patterns obtained with orthopoxviruses were generally quite similar to one another, but variola, monkeypox, cowpox and vaccinia viruses could be distinquished by their profiles in the molecular weight (mol. wt.) region around 30,000 to 40,000; some additional variation was found amongst cowpox and vaccinia strains. Whitepox virus was shown to have structural polypeptides indistinguishable from those of variola virus. The structural polypeptides of poxviruses belonging to other genera were different from those of the orthopoxvirus, except those of mol. wt. about 122,000 and 97,000, which were common to all viruses irrespective of genus. A polypeptide of mol. wt. about 25,000 was also observed in all cases, though its position varied slightly with the individual virus.