Abstract: A comparison of the experimental therapeutic activity against M, leprae of cyclopentylrifamycin SV (our code number R-77-3, or DL 473 by Lepitit) and RMP was performed in mouse footpad model. By continuous method, in which the drugs are administered once in four weeks, the EDso (the dosage required to prevent multiplication of M. leprae in the footpads of half of the mice) of R-77-3 was far below that of RMP. By using the proportional bactericidal method, the bactericidal activity of R-77-3 proved to be more potent than that of RMP. It is likely that the greater activity of R-77-3 has resulted from its much favourable pharmacokinetic properties, especially its much longer half-life in the body.These findings suggest that it may be more suitable to apply R-77-3 rather than RMP as intermittent therapy in leprosy.