Infection with Toxoplasma has been found to be widespread in the human population and, as a consequence, is receiving increasing attention. The clinical manifestations may range from inapparent infection or benign disease to severe systemic disease or a chronic stage with eye disease and encephalopathy and mental retardation, or both. Because of the variable nature of the disease and the high rate of inapparent infections, laboratory diagnostic methods are essential to proper diagnosis. Serologic tests have received a great deal of emphasis as diagnostic aids in suspected infections with Toxoplasma, the methylene-blue dye test being the most widely used. In recent years the indirect hemagglutination (IH) test has been reported to be a sensitive and specific serologic test that yields titers comparable to or slightly higher than the dye test. Because of its simplicity and safety to the laboratory worker, it has been recommended as a useful test for the serologic diagnosis of toxoplasmosis.