Levamisole was injected into cows six times at weekly intervals in the dry period and its effect on existing subclinical Staphylococcus aureus mastitis monitored through the following milking season. Levamisole therapy failed to eliminate or modify subclinical S. aureus mastitis. In a second experiment, a single levamisole administration at drying off did not prevent new experimentally-induced S. aureus infections in the first two weeks of the dry period and there were similar numbers of cases of clinical and subclinical mastitis in treated and control groups. However, levamisole may have reduced the severe clinical manifestations of the bacterial challenge, since gangrenous mastitis and abortion were only observed in the control cows.