The anti-Trypanosoma cruzi effect of allopurinol ribonucleoside and formycin B was examined against infections of the sensitive Y and Peru strains in inbred mice, strain DBA/1. Allopurinol ribonucleoside given in the drinking water at doses calculated to be 239, 511 and 929 mg/kg/day for 28 days, prevented the death of the mice but did not eradicate the infection. Formycin B given orally at 100 and 10 mg/kg/day X 5 days, showed a similar effect.