Toxoplasma antigen-specific lymphocyte transformation was measured in subjects in whom the diagnosis of acute acquired Toxoplasma infection was documented and in whom the time of onset of clinical illness was carefully determined. Neither false positive nor false negative reactions to Toxoplasma antigen were seen in lymphocytes from uninfected subjects or subjects with chronic Toxoplasma infection. Lymphocytes from subjects in both of these control groups responded equally to a different antigen, streptokinase-streptodornase (SK/SD). On the initial determination, the lymphocytes of 7 of 24 subjects with acute Toxoplasma infection of less than 12 months' duration failed to transform to Toxoplasma antigen. Five of these 7 subjects were among a group of 14 who had developed clinical illness less than 3 months previously. To determine when lymphocyte transformation to Toxoplasma antigen developed, longitudinal studies were carried out in subjects with acute Toxoplasma infection whose lumphocytes failed to transform initially to Toxoplasma antigen. It was found that in all patients, lymphocytes transformed with Toxoplasma antigen eventually, though in two patients this occurred between 9 and 12 months after the initial infection.