The rabbit complement components C3, Factor B, and Factor H were isolated and characterized and were compared to the corresponding proteins of human serum. Chromatographic behavior, chemical properties, and functional interactions show great similarities between the components in both species. By SDS polyacrylamide gel electrophoresis, the m.w. were estimated to be 195,000 for C3, 86,000 for Factor B, and 155,000 for Factor H. The amino acid compositions of the rabbit proteins resembled those of the human analog. The total carbohydrate content of rabbit C3 and Factor H was approximately one-half that of the human proteins. In addition, a qualitative difference in the carbohydrate moieties of the C3 proteins was observed. The serum concentration of the rabbit proteins was markedly lower than that of the human proteins. The rabbit C3b,Bb enzyme resembled the human analog with respect to half-life, control by Factor H, and stabilization by nickel ions.