The C3 phenotype distribution was investigated in different age-groups among 2,078 voluntary blood donors between the ages of 20 and 65 years, in a group of unrelated babies and in a group of old healthy persons. A continuous increase in the C3F gene frequency with age was found among the blood donors varying from 0.1780 in the youngest age group (babies: 0.1585) to 0.2516 at the age of 50–55 years followed by a continuous decrease to a level of 0.1700 among the eldest donors (0.1718 among the old persons). In the age group 45–49 years the C3 distribution differed significantly from that in the adjoining age-groups (C3F = 0.1619). It is believed that the variations are brought about by selection of the blood donor population and a balanced polymorphism for the C3 system, possibly due to differences in the biological efficiency of the C3 variants in the complement sequence.