Sera from mice immunized by repeated anthelmintic-terminated infections (IMS) or by a single primary infection (PMS) of Nematospiroides dubius were assayed for antibodies reactive with N. dubius antigens. The surface proteins of adult worms, the excretory/secretory (ES) proteins of adult worms and soluble extracts from lysates of both adult (AWH) and larval (LWH) N. dubius were used in an immunoprecipitation assay. A 60,000 MW protein was the major radiolabelled surface and ES protein. This antigen was dominant in precipitates by IMS from AWH, ES and surface-labelled worms but was not precipitated by PMS from any antigen source. Minor antigens of 20,000, 33,000, 36,000, 45,000, 50,000 and 66,000 MW were precipitated from AWH by both PMS and IMS but not from ES or surface-labelled worms. The dominant antigen precipitated from LWH by IMS was 20,000 MW. This antigen was not precipitated by PMS but larval antigens of 65,000 and 96,000 MW were precipitated by both PMS and IMS. The major antigens precipitated by IMS were adult (60,000 MW) and larvae (20,000 MW) stage-specific but some minor antigens (33,000, 45,000, 50,000 MW) were common to both stages. Our results show that the dominant antigen precipitated by serum immunoglobulin from mice immunized by repeated anthelmintic-terminated infections are proteins present on both the cuticle surface and in the ES.