In a comparison of in vitro properties of rifapentine (RIF) and rifampicin (RMP), the minimal inhibitory concentration of RIF against Mycobacterium tuberculosis in Tween-albumin liquid medium was usually 0.02 micrograms/ml, 2-3 times lower than for RMP; the bactericidal activity against a log phase culture was slightly less than that of RMP and the recovery after pulsed exposures to 1 microgram/ml of RIF or RMP lasting 6, 24 and 96 h was identical for the two rifamycins. These findings are used to interpret published data from the chronic experimental murine tuberculosis model and support the view that in the mouse, the efficacy of RIF in widely spaced intermittent chemotherapy is the result of its long half-life.