Cows with consistently high somatic cell counts (SCC) and bacterial infection, but no clinical signs of mastitis, were identified early in lactation. Some of these quarters were treated with Cloxacillin, while other similar quarters were left untreated. Prior to treatment, 74% of infected quarters had SCC higher than 300,000 cells/ml while 85% of uninfected quarters has SCC lower than 300,000 cells/ml. Infection was eliminated by treatment in five out of the eight quarters treated with Cloxacillin, and in these quarters SCC decreased from 4,200,000 cells/ml before treatment to 160,000 cells/ml after treatment. Milk production and composition were not affected significantly although production was about 14% higher in treated quarters than untreated quarters. The possible role of antibiotic therapy for subclinically infected quarters early in lactation was discussed in relation to a reduced rate of new infection in a herd.