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A simple hemagglutination inhibition (HI) test for the serological diagnosis of toxoplasmosis has been developed and evaluated. A total of 84 human and 120 mouse serum samples were tested by the newly developed HI test and compared with an immunoglobulin G-indirect fluorescent antibody test. Statistical analysis of serum titers obtained by using the HI test and the immunoglobulin G-indirect fluorescent antibody test showed a correlation coefficient of 0.89. The diagnostic efficacy of HI when compared with the immunoglobulin G-indirect fluorescent antibody diagnostic test results was 96.43% for human sera and 100% for mouse sera. The unique hemagglutination antigen, derived from Toxoplasma gondii (Rh strain) exotoxin, spontaneously binds with mouse or rat erythrocytes, causing the hemagglutination reaction. In this study, 2, 4, or 8 hemagglutinating units of T. gondii exotoxin was used with Swiss/Webster mouse erythrocytes as an indicator for the HI assay. The results indicate that 8 hemagglutinating units is optimal because this concentration has the least unexplained variability. T. gondii exotoxin was stable for at least 18 months at -70 degrees C. The Toxoplasma HI test we report in this paper is shown to be a fast, easy, highly specific, and sensitive test for the diagnosis of toxoplasmosis.