A subclinical bovine herpesvirus type 1 (BHV1) infection affected bulls at an artificial insemination centre for at least six months. The virus was detected in semen samples from 43 out of 116 bulls examined; 27 of them shed virus during one consecutive period of up to 10 days and 16 shed the virus intermittently. The virus titres ranged between 10 and 100,000 TCID50/ml. Vaccinated bulls tended to excrete the virus less frequently than unvaccinated bulls. Approximately 50 per cent of the bulls that had an increase in neutralising antibody titre to BHV1 shed the virus in their semen.