The in vitro-released 'excretory/secretory' (ES) and somatic antigens of the third stage (infective) larva of Anisakis simplex were characterised by radioiodination, immunoprecipitation, and sodium dodecyl sulphate-polyacrylamide gel electrophoresis. Oral infection with the parasite elicited a substantial IgG antibody response to ES in infected rabbits and humans, with a minimal response to somatic materials. Serial serum sampling in experimental infection showed that there was a sequential recognition of distinct ES components. In contrast to oral infection, intraperitoneal exposure of rats with living parasites induced a strong response to both ES and somatic antigen preparations. Sequential recognition of ES antigens, and differential responses to somatic components, might, therefore, have application in the estimation of the age and degree of penetration by the nematodes in human infection. Extensive antigenic relationships were found between A. simplex and three other species of ascaridoid nematodes, namely Ascaris lumbricoides, Ascaris suum, and Toxocara canis, but none with a panel of non-ascaridoid nematodes. Evidence is presented that a Mr 14,000 component of A. simplex has a homologue in all of the ascaridoids examined, but does not elicit an antibody response in anisakiasis. Finally, the ES of A. simplex is shown to contain two proteinase activities, of approximately Mr 23,400 and 46,100, as revealed by separation on gelatin substrate gels, although the antigenicity of the enzymes remains to be established.