Sera from 33 Australian thoroughbred mares were tested during an outbreak of equine herpesvirus 1 (EHV1) abortion with an enzyme-linked immunosorbant assay (ELISA) for the presence of EHV1-specific antibodies. The ELISA used a recombinant EHV1 antigen derived from glycoprotein G (gG) and distinguished antibodies to EHV1 from those of the antigenically related and widespread herpesvirus EHV4. Sera were obtained from most of the mares on three occasions, three, 13 and 67 days after the first abortion. Mares which were negative in the ELISA were kept separate from mares which were positive. A second abortion occurred two days after the first and two more abortions and one perinatal death occurred later. Sera from these last three mares showed a significant increase in EHV1-specific antibody on day 13 indicating a recent infection with EHV1. Ten other mares did not have antibodies to EHV1 on day 13 but had seroconverted to EHV1 by day 67. Despite the EHV1 infection, these mares foaled normally, possibly because the infection had occurred either late in gestation or after foaling. Seven mares that remained negative in the ELISA throughout the testing period did not abort, and neither did 11 mares that were positive in the ELISA when they were first tested.