A survey was made of the frequency of β-casein variants in the milk of cows of five major British breeds. Three variants A, B and C differing in electrophoretic mobility were found to occur either singly or in pairs, as expected if each were determined by a single allele. The A variant was the most common and found in all breeds. Ayrshires and Shorthorns produced no other variant, whilst the B variant occurred in Jerseys and, at low frequency, in Guernseys and Friesians. Production of the C variant was restricted to the Guernsey breed in which it occurred at low frequency. The technique of paper electrophoresis, used for β-casein ‘typing’ of some 600 cows, is described in detail.