Density gradient centrifugation using a performed self generated gradient of colloidal silica enabled the isolation of microscopic sheep sarcocystis cystozoites, free from heart muscle contamination. The efficiency of separation of cystozoites from residual heart muscle after digestion in pepsin and hydrochloric acid was 63 to 92 per cent. Antigens from cystozoites were used on enzyme-linked immunosorbent assays (ELISA) of plasma from six coccidia-free lambs infected once orally with 70,000 microcystic sheep sarcocystis sporocysts and for raising antisera in rabbits. Use of an anti-sheep IgM conjugate in the ELISA showed that anti-sarcocystis IgM production was transitory, appearing five to 10 days after infection, peaking in concentration at 42 days and following the peak of the acute phase of infection (32 and 33 days) in the lambs. In contrast, total anti-sarcocystis immunoglobulins, detected by ELISA, increased from five to 21 days after infection and continued to increase until the lambs were killed (the last at 81 days) and was more useful in diagnosing chronic infection. No cross reactions between microcystic sheep sarcocystis and Toxoplasma gondii or Eimeria species of sheep were observed. A peroxidase anti-peroxidase test, using rabbit anti-sarcocystis sera, detected second generation meronts and sarcocysts in fixed tissues from infected lambs making it useful for the diagnosis of acute or chronic disease post mortem.