The C3 types of human serum are reported for a material of 113 Danish cystic fibrosis patients, age 0-30 years. The frequency of the C3F gene was 0.2832 which was significantly higher (p less than 0.0005) than the frequency found in a control group of 224 healthy babies (C3F = 0.1585). It also differed significantly (p less than 0.01) from the C3F gene frequency of 0.1780 found in 177 blood donors, age 20-24 years. A significant association between any of the C3 phenotypes and the most serious infection in cystic fibrosis, chronic mucoid P. aeruginosa infection, or the antibody response against these bacteria was not found.