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Transduction of the lac + property by phage P1 among strains of Escherichia coli K12 and Shigella dysenteriae Sh has been studied. When E. coli strains are used both as donors and recipients, transduction gives stable lac + transductants (transduction by integration). Transduction from E. coli lac + donors to strain Sh gives mainly unstable, lac v transductants, in which the lac + genes are apparently associated with some phage P1 genes in a defective prophage, called P1 dl (transduction by lysogenization). A variety of P1 dl elements is postulated to account for the properties of different Sh lac v transductant strains. Some of these strains produce phage lysates that transduce the lac + property at high frequency (HFT lysates), probably because of a high content of phage particles carrying the defective phage element P1 dl . A different type of P1 dl element, called P1 dl w , has been obtained by using HFT lysates from Sh lac v strains to transduce the lac + property in a strain of E. coli that appears to have a deletion of the lac genetic region. The results of lac + transduction by various types of lysates to various recipient strains have been compared, using both untreated and ultraviolet-irradiated lysates. The results are interpreted by assuming that the transducing particles contain genetic elements with various combinations of phage genes and of bacterial genes, and that the transduction type that is obtained, by integration or by lysogenization, depends both on the ability of the transducing elements to lysogenize and on the degree of genetic homology between the donor and the recipient of the transduced bacterial genes. High genetic homology favors integration; low genetic homology hinders integration and allows detection of transductants carrying the transducing element as prophage.