A novel bovine beta-lactoglobulin W has been isolated and its complete primary structure is presented. It was isolated by chromatofocusing of a beta-lactoglobulin AW heterozygote and purified by recrystallization. During sequencing of the oxidized protein, it became evident that the new beta-lactoglobulin W is a subtype of variant B with a single difference at position 56. This Ile----Leu substitution, which means a shift of a methyl group from C-beta to C-gamma of the amino-acid side chain causes a change of pI of 0.007 units, which can be detected by high resolution electrophoresis. This Ile56 amino-acid residue is among the most conserved residues with the exception of kangaroo beta-LG. The structures of other bovine beta-lactoglobulins and their relationships are discussed.