The in-vitro sensitivity of 32 strains of Mycobacterium ulcerans to rifampicin was found to be similar to that of Myco. tuberculosis. A 1-mth course of rifampicin in three different dosage regimens was used to treat experimental Myco. ulcerans infection of mouse foot-pads starting 2 and 5 wk after infection. Each regimen led to clinical improvement, but the disease relapsed 6–8 wk after completion of therapy. Treatment of severe lesions for 10 wk produced apparent cure. A 1-mth course of sulphadoxine plus pyrimethamine or of clofazimine proved ineffective, and combination of these with rifampicin were no more effective than rifampicin alone.