Serum complement (C3, BF, C4) types were examined in Swedish Saamis (Lapps). In agreement with previous studies, a very low frequency of the C3*F allele (0.033) was found. Compared to Swedes and other Caucasian populations, Swedish Saamis showed a significantly increased frequency of the BFS, C4A4 and C4B2 variants, and a lower frequency of C4 deficiency. BFS, C4A4 and C4B2 show haplotype associations and thus the frequency of the S-A4-B2 complotype is significantly increased among the Saamis. Although the serum complement constitution of the Saamis shows some similarity with that of Asiatic Mongoloid populations it is unlikely to be due to Asiatic ethnic influence. The marked genetic deviations of the Saamis from all other populations has often been interpreted as a result of the founder effect and genetic drift. In this particular case, however, immunogenetic adaptation appears to be a plausible alternative explanation for the deviations in genetic complement factors.