Heat shock proteins (hsps) are a group of highly conserved polypeptides found in a wide variety of organisms. Polypeptides of sporozoites and merozoites of Eimeria bovis, blotted onto nitrocellulose, were probed with antibodies to artificially constructed peptides representing portions of the cDNA-generated fragment of pf75, the 75K hsp of merozoites of Plasmodium falciparum. Polypeptide antigens of sporozoites and merozoites of E. bovis with molecular weights of 46K, 71-72K, and 75K reacted with antibodies against pf75, indicating that they are hsp70 (the 70K family of hsps) or hsp70 cognates (noninducible proteins homologous to hsps). Radiolabeling with 125I and treatment with antibodies against pf75 detected a 71K antigen on the merozoite surface. Hsps in sporozoites of E. bovis are either constitutive or evoked by treatment at 37 C for in vitro excystation. If hsp70 is mandatory for parasite survival, it may prove to be an appropriate antigen for a vaccine against bovine coccidiosis.