Of 70 renal transplant patients submitted to the indirect immunofluorescence reaction test for toxoplasmosis, 16 (23%) had titers higher than 1/4,000, as compared to 0/41 for chronic hemodialysis patients and 0/50 for blood donors. The indirect immunofluorescence reaction titers in the renal transplant patients correlated with time since initiation of immunosuppressive therapy. Six percent (2/33) of the patients had been on immunosuppressors for less than one year, 30.7% (4/13) for 1 to 2 years, and 41.6% (10/24) for 2 or more years. The frequency of negative titers among the immunosuppressed patients was similar to that observed for blood donors and chronic hemodialysis patients. Fifty percent (8/16) of the patients with higher immunofluorescence reaction titers also had significantly high (greater than or equal to 320) positive titers in the complement fixation test. The results indicate that: 1) the immunosuppressive scheme used for the transplant patients may favor the reactivation of infection from latent Toxoplasma gondii foci, and 2) even though the patients were immunosuppressed, their antitoxoplasma antibody levels were high enough to be detected by the serologic test.