Plasmodium antigens were detected by dot-blot assay in the urine of 50 patients infected with Plasmodium vivax. Antigens also were detected in 12/15 patients who no longer had detectable parasitemia, 3 weeks after chemotherapy. Antigenuria was negative 6 weeks after treatment. By Western blotting, four predominant protein antigens were identified in the urine of patients infected with P. vivax: 200, 180, 150, and 110 kDa. The dot-blot technique may prove to be a rapid and inexpensive method for diagnosing malaria in field studies and for clinical evaluation during chemotherapy.