A number of different but complementary approaches have been used to demonstrate the immunogenicity of Necator americanus acetylcholinesterase to infected individuals. Western blotting of parasite somatic extracts with human post-infection sera and a specific rabbit antiserum to AChE resulted in the development of almost identical antigen-recognition profiles. AChE-containing fractions produced by preparative iso-electric focusing were subsequently shown to be antigenic in ELISA using post-infection sera. This preliminary data was reinforced by the affinity purification of AChE by immobilized post-infection IgG, and the immunoprecipitation of AChE activity from ES by post-infection IgG. Finally, AChE purified by affinity chromatography on edrophonium chloride was shown to be antigenic by Western blotting, and in ELISA, against post-infection sera, although a degree of re-activity was also seen with normal human sera. This data is discussed in the context of the host-parasite relationship.