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 seronegative. Border disease virus was detected in one lamb on day 19 and in another on days 19 and 154.