Superoxide dismutase has been characterized in normal and also in nitrofurantoin-resistant mutant strain of Vibrio el tor. An induction of this enzyme, which is noted in Vibrio el tor when grown under hyperbaric oxygen, is inhibited by nitrofurantoin. This induction of superoxide dismutase becomes insensitive to nitrofurantoin in Vibrio el tor after it acquires resistance towards the antibacterial agent. The lethal effect of superoxide radicals on Vibrio el tor is shown by both photo-chemically and enzymically generated superoxide radicals. Nitrofurantoin aggravated the lethal effect of superoxide radicals in normal as well as in nitrofurantoin-resistant mutant of the microorganism. Bovine superoxide dismutase preparation exhibited a protective effect on the enhanced lethality of superoxide radicals towards Vibrio el tor strains in the presence of nitrofurantoin. Superoxide dismutase could also inhibit the photochemical and enzymic activation of nitrofurantoin.