The C 3 types were determined in 56 Norwegian families with 172 children and in 1,220 Norwegian mother/child combinations. No exceptions from the rules of inheritance were observed. Variant C 3 types was seen in 3 families. The inheritance of the two common C 3 proteins was studied in a compiled family material comprising 152 Caucasian American, German, and Norwegian families with 376 children. No deviations from a normal inheritance pattern were observed. The present study revealed no indication of association between C 3 types and phenotypes within the ABO, Rh, MN, Hp, Gc or PGM1 systems.