Inactivation of homologous C3b by heated guinea pig, mouse and human serum was found to be much more rapid and complete at low ionic strength (0.037) than at micron = 0.15. The C3b inactivator in human and mouse serum was somewhat unstable to heating at 56 degrees C. Heated guinea pig serum showed the greatest ability to inactivate heterologous C3b, and human serum the least. Suramin (1 mg/ml) completely blocked homologous C3b inactivation by heated human, guinea pig and mouse serum, and 0.1 mg/ml was effective with mouse but not with human or guinea pig serum. Immune-adherence reactions with mouse C3 produced somewhat unstable hemagglutination patterns, which were improved by using ovalbumin in the buffer and minimizing EAC exposure to warm temperatures. A prozone phenomenon was frequently observed in immune-adherence hemagglutination with mouse C3, and less frequently with guinea pig and human C3.