Evidence of seroconversion and transient viraemia were observed in eight healthy lambs housed for 154 days with a lamb persistently infected with border disease virus. Blood was collected from the jugular vein on days 1, 19, 33, 48, 60, 82, 103, 126 and 154 for the detection of pestivirus antigen and antibody titres. All lambs remained clinically healthy throughout the study period. Seroconversion was observed in six lambs, one had a questionable titre and the remaining lamb remained serone-gative. Border disease virus was detected in one lamb on day 19 and in another on days 19 and 154.