Three casein loci, tightly linked on bovine chromosome 6, have been studied as haplotypes within families of bulls. The polymorphisms included in the study were CASAS1 (B, C), CASB (A1, A, A5, B), CASK (A, B, E) and a microsatellite in intron III of CASK. A new A5 variant of CASB, caused by a silent mutation in the triplet coding for amino acid 110, was detected by direct polymerase chain reaction sequencing. Our analysis of 306 sons and 15 sires revealed 10 different casein haplotypes with a cumulative polymorphic information content (PIC) of 0.78.