The 202,000 MW protein XC, which is distinct from C3 but is essential for antibody-dependent haemolytic activity against SRBC, was obtained from Xenopus plasma following polyethylene glycol precipitation, ion-exchange chromatography and gel-filtration. The protein required other components of the Xenopus serum in order to lyse sensitized SRBC, but did not lyse unsensitized RRBC through the alternative pathway. The protein, contained at 0.17 mg/ml in the original plasma, comprised three distinct subunits of 96,000, 76,000 and 26,000, which were linked by disulphide bonds. Digestion by trypsin resulted in a specific cleavage of the 96,000 subunit and a conversion of its immunoelectrophoretic mobility to the anodal side, leaving the 76,000 and 26,000 subunits intact. The treatment with SDS and urea resulted in the splitting of the 96,000 subunits into 48,000 and 45,000 components, but this splitting was inhibited upon pretreatment with methylamine, suggesting the presence of a thiol ester bond in the 96,000 subunit. The amino acid composition of the XC revealed a striking resemblance to that of human C3 and C4. We therefore conclude that the 202,000 protein isolated in this study represents the C4 which plays an essential role in the classical haemolytic pathway.