1. Heterologous reactions of mammalian serum proteins with monospecific anti-human sera were investigated by use of the Ouchterlony technique in an attempt to further elucidate the biochemical evolution of human serum proteins. Sera from chimpanzee, rhesus and mangabe monkeys, pavian, cattle, sheep, pig, guinea pig, rats (Wistar and Sprague-Dawley) - and for comparison those of chick and eel - were examined for common antigens. 18 monospecific antisera were used including sera against human albumin, ceruloplasmin, transferrin, hemopexin, haptoglobin α1 and β-lipoproteins, Lp(a)-factor, IgG, IgA, IgM, κ{script}- and λ-chains, a γG-paraprotein, β1C-A-globulin, α2HS-glycoprotein, α2HS-glycoprotein, α2 and the Gc-factor. 2. All primate sera reacted with all anti-sera tested excluding anti-IgA,-IgM,-κ- and-λ-chains. Anti-IgA and-IgM, however, reacted also with the serum of the chimpanzee. 3. All or most of the non-primate mammals reacted with anti-human albumin, ceruloplasmin, LDL and α2, two mammals reacted with anti-α1-lipoprotein and β1C-A-globulin. In the case of IgG a serum with an especially high titre was necessary to get a reaction against 3 non-primate mammals. All the other antisera including those against human transferrin, hemopexin, haptoglobin, IgA, IgM, L-chains, 12HS-glycoprotein and the Gc-factor failed to react with non-primate mammalian sera. 4. The sera of both chick and eel did not cross-react with human serum. 5. The minimum number of antigenic determinants of human LDL was calculated to be 6 (excluding those belonging to the Lp-, Ld-, and Ag-factors). 6. The height of the titres as compared with those obtained with human serum was found to be dependent from the phylogenetic relation between man and the resp. species. 7. To compare immunological resp. molecular-biological and morphological data on the human evolution directly, the titres of the heterologous reactions and the LDL-determinants were drawn into a scheme showing the vertebrate evolution as derived from morphological findings. A clear-cut parallelism between immunological and morphological evolution is found this way. Relations between immunological cross-reactions and biochemical evolution are discussed in greater detail. © 1969 Springer-Verlag.
