ELISAs to detect antigens against Staphylococcus aureus, Escherichia coli, Streptococcus agalactiae, Streptococcus dysgalactiae and Streptococcus uberis were developed and used on whey samples from quarter milk samples from cases of clinical mastitis in which no bacterial pathogen had been isolated by standard culture techniques. Antigens against one or other of the bacteria listed above were detected in 68 per cent of 84 samples. Fifty-one per cent of the 84 samples contained antigens to E coli. Antigens against one or other of these bacteria were also detected in 51 per cent of 53 quarter milk samples which were negative on culture but had somatic cell counts of more than 500,000 cells/ml. It is suggested that ELISAs for the detection of antigens in quarter milk samples could be a useful adjunct to culture, to determine the infection status of a quarter.