Murine Ss protein was found to exist in two forms, distinguishable by electrophoresis, ion exchange chromatography, and gel filtration chromatography. Mouse blood collected directly into EDTA yielded electrophoretically slow Ss protein which eluted from DE-52 cellulose in fractions that also contained C3 and eluted from Sephadex G-200 in fractions containing molecules of 160,000 daltons when high ionicity buffers were used. Mouse serum yielded an electrophoretically faster Ss protein that eluted from DE-52 cellulose well after C3 and eluted from Sephadex G-200 in fractions containing molecules of approximately 128,000 daltons when buffers of high ionicity were used. Incubation of serum led to increasing proportions of the smaller form with time. With appropriate conditions, Ss protein antigen could be separated from C4 functional hemolytic activity on ion exchange chromatography and gel filtration. EAC142 that had been generated by incubation of EA in fresh B10.D2 new serum lost all functional C1, C4, and C2 but not Ss protein from their surface when incubated in EDTA-containing buffers. Treatment of mouse serum with the anti-complementary factor of cobra venom left the C1, C4, and C2 titers unchanged but caused the serum to lose its ability to generate cells that could be agglutinated with anti-Ss antiserum. Similarly treated human serum retained its C1, C4, and C2 activity and its ability to generate cells agglutinable with anti-C4 antiserum. Treatment of mouse serum with NH4OH caused only minimal loss of C4 function but caused the serum to lose its ability to generate cells that could be agglutinated with anti-Ss antiserum. Similarly treated human serum lost both C4 function and the ability to generate cells that could be agglutinated with anti-C4. Thus, mouse Ss protein, an antigenic homologue of human C4, was shown to have several functional and physicochemical differences from human C4. Genetic, evolutionary, and immunologic aspects of this are discussed.