The relationship between anti-parasite IgA antibody levels in plasma and the periparturient egg rise in sheep was investigated. Ostertagia circumcincta larvae (5000 third stage larvae three times weekly) were administered to three groups of seven adult immune ewes from 12 weeks before until three weeks after lambing (group 1) or from six (group 2) or 14 (group 3) weeks before until three weeks before lambing. Seven additional ewes were not challenged (group 4 controls). Ewes in groups 1, 2 and 4 received anthelmintics 14 weeks before lambing. Challenge of the pregnant ewes with O circumcincta larvae resulted in substantial increases in faecal egg counts only during the periparturient period regardless of the larval dosing regimen. Furthermore, the periparturient rise in faecal egg counts was closely associated with a significant increase in anti-parasite IgA antibody levels in plasma. This rise in IgA antibody levels occurred at a time when IgA is transported from the gut to milk during early lactation. It is postulated that this may lead to a temporary reduction in abomasal antibody levels of ewes and hence permit the establishment of larvae and, or, the emergence and development of inhibited larvae and thereby lead to the periparturient rise in faecal egg count.