Abstract: Isospora suis is propagated using a swine testicular cell line which facilitates production of sporozoites and merozoites, either or both of which can be used in a vaccine for swine. Antibodies against Isospora suis sporozoites, which inhibit the infectivity of the sporozoites for the swine testicular cells, were made. These neutralizing antibodies were used to identify an apparent sporozoite attachment protein and cloned cDNA encoding a portion of the protein. The cloned cDNA was sequenced and the corresponding amino acid sequence of the antigenic sporozoite protein revealed two areas having repeated amino acid sequences, which may likewise be used in a vaccine.