Sera from 32 patients who became ill after jungle combat training were tested for antibodies to Toxoplasma gondii using the indirect immunofluorescence test. Swift rises of both IgG and IgM antibodies occurred within 2 weeks of infection. Reduction in IgM titers, due to competitive suppression by IgG antibody, occurred in most but not all cases. Suppression of IgM reaction by IgG antibody could be prevented by adsorption of serum with Staphylococcus aureus containing protein A. Antibody of the IgM class could be detected at ≥ 1:256 level in many sera at 6-month and 1-year intervals after exposure. In groups with exposures such as were experienced in this study, the presence of IgM antibody titers in single serum specimens cannot be used to indicate recent exposure. Both IgG and IgM antibody may rise together to high levels very rapidly after infection; IgM did not precede IgG antibody in our 32 subjects.