SummaryGenetic restrictions to the immune repertoire will be an important consideration in the development of anti‐nematode vaccines. It has already been established that the major histocompatibility complex (MHC) limits responsiveness to nematode antigens in infection, but little is known of whether this also applies under other routes of sensitization, such as with adjuvants. The specificity of the antibody response was, therefore, compared in infection and adjuvant‐assisted immunization using secreted and somatic antigens ofAscaris suumas a model system in mice and rats. The findings were, first, that the lack of responsiveness to certain antigens in infection was not circumvented by Freund's adjuvant‐based immunization, despite the fact that the latter generally elicited higher levels of response. Secondly, that adjuvant‐assisted immunization could elicit responses to parasite products which were not detectable in the context of infection. Conversely, some specificities were detectable in infection but absent under adjuvant immunization. Finally, immunization with a defined parasite allergen (ABA‐1) in Freund's adjuvant did not provoke an IgE response which would be anticipated if the molecule were to have an intrinsic allergenic property. These results are likely to be of general importance to the application of subunit or recombinant vaccines against nematodiases and to the hypersensitivity reactions which vaccination might engender