Animal-derived hydatid-fluid antigens, principally Australian commercial antigen, were skin-tested in Puerto Ricans, other persons of Caribbean origin, and in native-born New Yorkers. The findings suggested that there existed a certain previously unreported degree of cross-reactivity to Casoni antigen in persons with schistosomiasis and possibly in nematode parasitic infections. Geographic matching of controls with cases would, therefore, be necessary in order to determine the usefulness of parasitic skin-test antigens, and the results of population surveys with these antigens would have to be interpreted cautiously. Interpretation of Casoni skin tests with commercial antigens must be made in the light of cross-reactions in persons exposed to other helminth infections. A human-derived antigen, and an animal-derived antigen of low nitrogen content, did not appear to possess any considerable advantage over those that are commercially available.