The subunit composition of different electrophoretic forms of canine C3 and C4 was determined by sodium dodecyl sulfate-polyacrylamide gel electrophoresis analysis of reduced immune precipitates. Canine C4 comprises alpha, beta, and gamma chains of approximate molecular weight 90 000-95 000, 72 000, and 33 000, respectively. The molecular weight of the alpha chain of the C4 1 allelic product was approximately 95 000, but 90 000 for the C4 2 and C4 4 allotypes. No differences were observed in the molecular weights of the beta or gamma chains of any canine C4 phenotype tested. Canine C3 appears to be encoded by a single locus. The subunit composition comprises an alpha and beta chain with molecular weights of approximately 106 000 and 71 000, respectively. Unlike C4, no differences in the molecular weights of the subunits were observed in different electrophoretic forms of canine C3.