Dapsone (4,4'-diaminodiphenylsulfone), incorporated into the mouse chow in a concentration of 0.1 g/100 g of diet, was administered for 1 week to mice in which Mycobacterium leprae had multiplied to the level of 10(6) organisms/footpad. M. leprae were harvested from these and also from control mice, diluted serially, and inoculated into additional mice. The organisms recovered from untreated mice multiplied in passage with a mean doubling time of 12.2 days, and 35% or more of the inoculated organisms were viable, i.e., capable of infecting mice. Growth curves of M. leprae recovered from dapsone-treated animals lagged behind those of organisms from control animals by an average of 78 days, equivalent to 98.8% killing. Foot-by-foot harvests showed that only 0.2% of the M. leprae recovered from treated mice were viable, suggesting that treatment of mice with dapsone had been accompanied by killing of 99.4% of the viable M. leprae.