Summary Plasma transferrin (Tf) types in dogs were studied by a method of horizontal polyacrylamide gel electrophoresis with a 10 % separation gel and a discontinuous buffer system (Tris-citrate-borate, pH 9.0). Samples were analysed from 1127 dogs belonging to 60 different breeds. TfB and Tfc alleles, each in considerably high frequency, were observed in most of the breeds. TfA with relatively lower frequency was observed in Beagle, Cocker Spaniel, Doberman Pinscher, German Shepherd, German Shorthaired Pointer, Old Danish Pointer and Poodle. Two rare Tf alleles, TfD and the other designated as TfE, were observed only in Cocker Spaniel and in Poodle respectively. Basenji (44 samples) and Small Munsterlander (17 samples) showed the highest frequency (0.97) for TfC allele while Carelian Bear Dog (19 samples) showed the highest frequency (0.97) for TfB allele. In the total material, the frequencies of Tf alleles A, B, C, D and E were found to be 0.0182, 0.5075, 0.4716, 0.0009 and 0.0018 respectively. By using the observed and expected numbers of Tf heterozygotes, the average inbreeding coefficient (F) within breeds was estimated to be 0.14. Six samples of wolf (Canis lupus L.) studied seemed to be of Tf B type. The present study, along with earlier reports, showed that some of the plasma proteins and enzymes (albumin, transferrin, eserine resistant esterase) exhibit considerably more polymorphism than that reported for hemoglobin and some of the red cell and tissue enzymes in the domestic dog.