Abstract: The prooprtional bactericidal test(PBT)can be used to assess and compare the bactericidal acjivity of drugsagainst M.Leprae through the estimation of the most probable number(MPN)of viable bacilli and the number of AFB necessary to infect 50% of the mice, i.e. ID₅₀ The bactericidal activity of two new rifamycin SV derivatives, i.e, isobutyl-piperazinplrifamycin SV(R-76-1)and methyloxime of 3-formylrifamycin SV(AF-MO),was significantly superior than that of rifampicin and dapsone by PBT.