Two hundred cases of mild clinical mastitis were treated on two farms using two intramammary preparations in quick release bases. One preparation contained penicillin and streptomycin, the other contained lincomycin, neomycin and prednisolone. Fifty-eight per cent of cases were clinically and bacteriologically cured using the first preparation; and 61 per cent of cases, with a similar range of organisms, using the second. The main infections were Streptococcus uberis and coliform; all but three of the infections were sensitive in vitro to one or both of the antibiotics in the preparations. On the farm with 100 cows, 69 per cent of cases were cured, while only 52 per cent were cured on the farm with 300 cows. It is suggested that it is more difficult to detect and treat cases in the larger herd. A number of reservations about the interpretation of the results are discussed.