Population studies have shown useful correlations between alleles at the casein loci and important production parameters such as compositional characteristics of milk, cheese yield and lactation yield. Molecular genetic analysis of genotype at the casein loci could therefore enhance sire selection in dairy herds. We are Investigating polymorphisms in K-casein, where the B-variant gives improved cheesemaMng and in β-casein, where the A3 allele is associated with improved lactation yields. We have previously reported a restriction fragment length polymorphism in the K-casein gene, enabling us to identify the alleles at this locus [Perry, B.N., Savva, D., Radley, E., Skldmore, C.J. & Lovell, R.D. (1989) Anim. Prod. 48, 661]. Our current work makes use of the polymerase chain reaction (PCR) to increase dramatically the speed of di agnosis.