The cross-reactivity of the immunoglobulin γ, γ 1 , γ 2 , α, μ, and light chain antigens of 21 ruminant species was examined using an inhibition of radioimmune precipitation technique. Although it was clear that the different antigens had evolved at different rates, the relative ranking of cross-reactivity correlated well with the accepted taxonomic order of the ruminants. It was considered significant that Antilocapra was closer to the bovids than to the cervids, in view of the disputed relationship of that genus to these families. Further, it was noted that sheep and goats appeared to be no more or less closely related than, say, the cattle and the antelopes. Of the IgG subclass antigens, IgG 1 showed the higher evolutionary stability, strong cross-reactivity being observed between sheep and deer and even between sheep and camel. This stability might be due to the conservation of sequences related to the biological properties of IgG 1 —its ability to fix complement and its secretion into the colostrum and absorption in the gut of the newborn.