The serum β-globulin types of 360 cows in fifteen herds of nominally Jersey cattle in the Nambour region of Queensland and 423 cows from nineteen herds of nominally Australian Illawarra Shorthorn cattle in the Kingaroy region were determined, together with the β-globulin types of eighteen Jersey bulls used for artificial insemination in the Nambour region and nine Australian Illawarra Shorthorn bulls used in the Kingaroy region. The results, expressed as breeding efficiencies, of 1527 inseminations from the Jersey bulls and 1166 inseminations from the Australian Illawarra Shorthorn bulls were then examined with respect to the β-globulin types of the bull and cow. It was found that β-globulin type had a highly significant effect on fertility in both regions. In the Nambour region, the breeding efficiency for matings between partners both homozygous at the β-globulin locus was 57·98% compared with 47·68% (X2 = 11·28, P<0·001) for partners one or both of which were heterozygous. In the Kingaroy region, the comparable breeding efficiencies were 62·93% and 47·24% (X2 = 12·83, P<0·001). In each region, the breeding efficiency with the homozygous bulls was about 4% greater than the mean for the region. The practical significance of these observations is discussed.