During a study of sarcosporidiosis in man, rats too were fed on meat infected with Sarcocystis tenella and A. fusiformis. Further investigations were made on the antigen relationship between Sarcocystis and Toxoplasma using 50 mice infected with Toxoplasma. The results are as follows: 1. 18-39 days after eating raw minced meat (beef) containing Sarcocystis fusiformis, 5 (=41,7%) out of 12 people excreted sporulated sporocysts of the species Isospora hominis in the faeces. 2. Excretion of sporocysts lasted for 9-179 days without reinfection. None of the above mentioned people showed any clinical symptoms. 3. There was no manifestation of immunity as the excretion of sporocysts which had reduced in number intensified again after repeated administration of similar raw minced meat. 4. Oral administration of Sarcocystis as antigen leads to the production of antibodies which could be detected with the indirect immunofluorescent test (IIFT). These antibodies are common among adults but could be detected in children after 6 months of age. 5. Sarcocystis antibodies produced in rats after ingestion of fresh raw S. tenella-infected meat could be detected with the help of the IIFT in their sera for several months. 6. The indirect immunofluorescent test is reliable in detecting Sarcocystis infection. 7. No cross-reaction between Sarcocystis and Toxoplasma was detected in either the human sera or in the experimentally Sarcocystis-infected rats and Toxoplasma-infected mice. 8. Parasites lose their capability to function as antigen when subjected to heat or cold because rats fed on frozen or cooked S. tenella-infected mutton did not produce antibodies compared to those fed on fresh raw infected mutton. 9. Sera of rats fed on different types of sausages reacted negative with Sarcocystis antigen.