When microcin B17-immune cells are treated with microcin B17 they show many of the physiological effects displayed by microcin B17-sensitive cells treated in the same way. DNA replication stops immediately and several SOS functions are subsequently induced. In sensitive cells these effects are irreversible and lead to cell death, whereas in immune cells they are reversible and there is no loss of viability. This is an unusual mechanism of immunity because it does not prevent the primary action of the microcin. The implications of this mechanism concerning the mode of action of microcin B17 and the induction of the SOS system are discussed.