The California mastitis test (cmt) is used on farms to identify subclinical mastitis by an indirect estimation of the somatic cell count (scc) in milk. Four commercially available detergents were compared with a bespoke cmt fluid for their ability to detect milk samples with a scc above 200,000 cells/ml; differences between the interpretation of the results of the tests by eight operators were also investigated. The sensitivity and specificity of the test were affected by the type of detergent, and by the operators' interpretations. When used by the most sensitive operator, suitably diluted Fairy Liquid performed almost identically to cmt fluid in identifying milk samples with more than 200,000 cells/ml. The average sensitivities achieved by the eight operators for detecting this threshold were 82 per cent for Fairy Liquid and 84 per cent for cmt fluid, and the specificities were 93 and 91 per cent respectively. The other detergents contained less anionic surfactants and were less sensitive but similarly specific.