The antibody response to Toxoplasma gondii was studied in human serum samples obtained before and during acute infection with T. gondii. Analysis was by the Sabin-Feldman dye test, double-sandwich IgM ELISA, agglutination test, and protein blots of toxoplasma antigens. Seventeen patients were studied: 12 pregnant women, 2 heart transplant recipients, and 3 symptomatic but immunologically normal individuals. Both IgM and IgG protein blots of the first serologically positive serum sample from each patient revealed new bands, as well as intensified staining of bands noted with the serologically negative serum samples. In each patient, bands corresponding to antigens of approximately 4 kilodaltons (kDa; IgM blots) and of approximately 35 kDa (IgG) were the most intensely stained. In one patient, IgM antibodies to the 4-kDa antigen(s) were demonstrable by protein blots before they were demonstrable by IgM ELISA. IgG protein blots of sera from individuals with chronic infection with T. gondii revealed weakly reactive antibodies to both the 4-kDa and 35-kDa antigens; these bands were not demonstrable in the IgM blots.