The distributions of plasma concentrations of complement proteins C3 and C4 were studied in sample populations of merino and Suffolk sheep. No differences between the breeds or the sexes were observed. The distribution for ovine C4 was polymodal and very disperse relative to that for C3. It was found, however, that C3 concentrations were elevated in specimens from 20 merino sheep bred as high responders to a Trichostrongylus vaccine. Significantly decreased plasma C4 concentrations were observed in representatives of both merino and merino X Border Leicester cross-bred sheep affected with congenital progressive ovine muscular dystrophy. Agarose gel electrophoretic variants of ovine C3 were not detected. Evidence for electrophoretic variants of ovine C4 in agarose gels was found although individual allotypes could not be reliably identified. Sodium dodecyl sulphate polyacrylamide gel electrophoresis (SDS-PAGE) did not reveal size heterogeneity for the alpha and beta chains of immunoprecipitated ovine C3. Analysis of reduced immunoprecipitated ovine C4 by SDS-PAGE revealed considerable size heterogeneity in the alpha chain consistent with isotypic and/or allotypic variability. The data presented strongly suggest the presence of two C4 loci in sheep, each of which exhibits polymorphism.