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All the quarters in the cows with high somatic cell counts in 10 herds were treated at drying off with either 600 mg cloxacillin or 600 mg cloxacillin and 4 g of an internal teat sealant containing 65 per cent bismuth subnitrate. The quarters were sampled daily for bacteriological tests for the three days before drying off and twice after calving to establish whether they were infected. Clinical mastitis cases were monitored from drying off until 100 days after calving. The odds of a quarter being bacteriologically negative after calving or developing clinical mastitis in the first 100 days after calving were investigated by multilevel logistic regression. The quarters treated with the internal sealant and cloxacillin were significantly more likely to be bacteriologically negative in the immediate period after calving and were significantly less likely to suffer clinical mastitis during the first 100 days after calving than the quarters treated with cloxacillin alone. There was more variation between cows than between herds in the underlying risk of an infection after calving, but there was more variation between herds than between cows in the underlying risk of clinical mastitis during the 100 days after calving.

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.

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