The ability of lymphocytes from 29 subjects to transform in response to Toxoplasma gondii antigen was studied. Cells from persons who had serologic evidence of toxoplasmosis, as manifested by elevated titers (of at least one year's duration) in the dye test and/or indirect IgM fluorescent-antibody test, transformed more frequently than lymphocytes from negative individuals. No correlation was found between the level of transformation and the dye-test titer. The level of transformation in four of six patients who were suspected of having acute toxoplasmosis did not differ from that of the control group. These data indicate that the technique of lymphocyte transformation may be applied to the study of infections of man with T. gondii and may be valuable in studies of antigen recognition in immunosuppressed patients.