The surface membranes of Eimeria tenella sporozoites were labelled with 125I and polypeptides resolved by polyacrylamide gel electrophoresis in sodium dodecyl sulphate (SDS-PAGE). The most heavily labelled polypeptides were 47, 26, 21 and less than or equal to 18 kDa but significant amounts of 125I were incorporated into a number of polypeptides with molecular weights ranging from greater than 200 000 to less than 18 000. Similar 125I-polypeptide profiles were observed after the surface labelling of sporozoites of E. acervulina, E. maxima and E. nieschulzi. Sporozoites of E. tenella were also radiolabelled by incubation in medium containing [35S]methionine and SDS-PAGE resolved more than 35 radiolabelled polypeptides with molecular weights from greater than 200 000 to less than 18 000. 125I and 35S-labelled sporozoites of E. tenella were solubilised in the detergents Triton X-100 or sodium deoxycholate and immunoprecipitated with serum from chickens immunized by infection with this species. Polypeptides of unlabelled E. tenella sporozoites, resolved by SDS-PAGE, were blotted onto nitrocellulose and the antigens, which reacted with the chicken serum, identified by immunoperoxidase staining. There was some variation between different sporozoite preparations in the number and molecular weights of antigens identified by these techniques but, consistently, the major surface polypeptides that were specifically immunoprecipitated were 104, 47 and 43 kDa. Specifically immunoprecipitated 35S-labelled antigens were of 123-94 kDa, 54-42 kDa and 32-25 kDa and antigens detected on Western blots were within the following ranges: 113-96 kDa, 73-67 kDa, 54-42 kDa, 37-32 kDa and 18-14 kDa.