The prophylactic effect of 2-sulfamoyl-4,4′-diaminodiphenylsulfone (SDDS) against Toxoplasma was studied in artificially infected pigs. Sixteen pigs weighing 12 to 24 kg whose serum was negative for Toxoplasma antibodies by the dye test were divided in six groups of two to four animals. SDDS was added daily to their food from 3 days before intraperitoneal inoculation of 2 × 107 trophozoites of Toxoplasma (porcine strain HG) at daily doses of 0 (control), 1.25, 2.5, 5, 10, and 20 mg per kg of body weight. The medication was continued for more than 40 days or until the animal was killed. Infection with Toxoplasma was completely prevented in pigs that received 5 mg or more of SDDS per kg per day, as determined by the absence of clinical symptoms, parasitemia, and hemagglutinating antibodies in the serum throughout the experiment and by the absence of the parasite in the tissues on post-mortem examination. In pigs receiving the lower dosages, the proliferation of the parasite was suppressed, but the infection was not completely prevented.