An assay was developed and evaluated for screening for Staphylococcus aureus in milk samples from cases of bovine mastitis by overnight cultivation in a broth containing 7-5 per cent sodium chloride, followed by PCR to amplify the nuc gene. The assay could detect concentrations of S aureus as low as 1 colony-forming unit/ml milk. Among 106 milk samples collected from individual quarters of lactating cows in one dairy herd and from a bulk tank, S aureus was detected in nine samples by the PCR assay but in only three samples by conventional microbiological culture.