The bactericidal activity of haemocin, a bacteriocin produced by Haemophilus influenzae b, for Escherichia coli CR34 could be blocked by the addition of 2,4-dinitrophenol. No inhibition by 2,4-dinitrophenol of the haemocin activity on H. influenzae Rd strain C25 was demonstrated. The data suggest an energy-dependent step in the mode of action of haemocin on strain CR34.