The rate of incorporation of hypoxanthine was measured in suspensions of Mycobacterium leprae, with and without added anti-leprosy agents. Dapsone, clofazamine and brodimoprim, as well as other benzylpryimidines, inhibited hypoxanthine incorporation, and their minimum inhibitory concentrations for incorporation with intact M. leprae were near the minimum inhibitory concentrations at which the agents have antibacterial effects. At sub-inhibitory concentrations for hypoxanthine incorporation, some combinations of benzylpyrimidines and dapsone were inhibitory, suggesting that synergic effects of anti-leprosy agents might also be detected by the inhibition of hypoxanthine incorporation. Thus, demonstration of inhibition of hypoxanthine incorporation in M. leprae could be a rapid method for screening anti-leprosy agents and especially for preliminary testing of new, potential anti-leprosy agents. The rate of hypoxanthine incorporation was generally lower in suspensions of M. leprae with lower viability, but it was not proportional to viability so the technique would not be suitable for accurate determination of viability.