A modified electrophoretic system for the determination of C4 polymorphism has been found with which the new allotype FI could be detected. The system has been applied to the population distribution of C4 in 266 unrelated Germans, further to association and linkage studies. Gene frequency was 0.3985 for C4^F, 0.5526 for C4S and 0.0489 for the rare C4 genes. In the population sample, significant association between Bf and C4 but no indication for close association between C4 and GLO I has been found. In the families HLA, Bf and C4 segregated together.