The recognition of parasite molecules from third-stage and adult Ostertagia circumcincta by serum antibody was studied in a group of matched, mature Scottish Blackface sheep that had been naturally and then deliberately infected. A total of 20 molecules was recognized in somatic extracts from third-stage larvae and 31 molecules in somatic extracts from adult parasites. However, no sheep recognized all immunogenic molecules and no molecule was recognized by all sheep. There was no obvious relationship between recognition of any parasite antigen and polymorphism at class I loci or at the DRBI class II locus of the major histocompatibility complex in these outbred animals. Only 15 molecules from third-stage larvae were present at a frequency suitable for statistical analysis and recognition of three of these 15 molecules was associated with differences in worm burdens. Recognition of two of five molecules from adult parasites was associated with differences in worm length. These results indicate that variation in the recognition of specific, identifiable parasite molecules may be partly responsible for variation among sheep in resistance to O. circumcincta.