The accumulated and de novo synthesized antigens expressed by L3, L4 and adult Necator americanus, recognized by both the natural host, man, and the experimental host, the hamster, were identified by immunoblotting and immunoprecipitation analysis. Following infection of neonatal hamsters serum samples were taken on days 17, 35 and 117. Only serum taken 117 days after infection showed significant reactivity in immunoblotting experiments, recognizing adult epitopes of 30,000, 33,000, 48,000 and 69,000 mol. wt thereby suggesting that few accumulated antigens are shared between developmental stages. By contrast, immunoprecipitation analysis of metabolically labelled proteins suggested that L3 and in particular L4 larvae synthesize some antigens which comigrate with those synthesized and accumulated by adult worms. In addition, L4 larvae synthesize a 41,000 mol. wt excretory/secretory (ES) stage specific antigen. Parallel experiments using serum samples from infected humans, demonstrated that hamsters and man recognize many antigens of identical molecular weight. Notable in this respect are accumulated adult antigens of 30,000, 33,000, 48,000 and 69,000 and de novo synthesized antigens of 30,000, 33,000, 44,000, 46,000 and 69,000 mol. wt. Some individual human sera mainly recognized L3 antigens of 47,000-69,000 mol. wt in immunoblotting experiments whilst others simultaneously recognized adult epitopes. This differential recognition of developmental stages by individual human sera suggests that genetic or epidemiological factors are operative and warrants further study. Overall, these studies confirm the pronounced immunogenicity of Necator americanus in both man and an animal model and pave the way for analysis of the relevance of these antigens to field situations.