A method of ELISA diagnosis of tertian malaria based on detecting Plasmodium vivax antigen in red blood cells (RBC) was developed, using 0.16 ml of packed, washed, and sonicated RBC. 68 blood samples from tertian malaria cases were examined; 67 (98.5%) were positive. 104 normal persons were all negative by this test. The lowest parasite number detected was 3 parasites/10(5) RBC on a thin film, or 1-2 parasites per thick film of usual size.