The preparation and specificities of a blood typing reagent, designated Tx, which defined individual differences in tahr goats, the homologous species, are described. It is shown that the red cell antigens involved in the reactions with the Tx reagent are the same as those which cross react with classical Forssman antibodies prepared by immunizing rabbits with extracts of guinea pig organs. Data are presented on the distribution of Forssman blood factors in representatives of 33 species in the order Artiodacyla. Individuals with Forssman-positive red cells were encountered in only 1 family, namely, the Bovidae, and within that family they were confined to 3 of 5 subfamilies, namely, the Hippotraginae, the Caprinae and the Ovinae. Three subtypes of the Forssman or Tx blood factor are described: subtype 1 which was observed in all species of caprines (subfamilies Caprinae and Ovinae) represented in the study; subtype 2 which was observed in only 1 species, namely, the blesbok antelope of subfamily Hippotraginae; and subtype 3 which occurred in only 1 species, the white bearded gnu of subfamily Hippotraginae.