An oxidation product of indole-3-acetic acid, 3-methyleneoxindole, is highly reactive with free sulfhydryl groups. It has been shown to be a potent bactericidal agent in actively growing cultures. The bactericidal action of 3-methyleneoxindole was found to be independent of protein or deoxyribonucleic acid synthesis but to require the capacity for ribonucleic acid synthesis. The reactions underlying the bactericidal effect of 3-methyleneoxindole are not unique to it among compounds which can bind free SH groups, since N -ethylmaleimide appeared to have a similar mode of action.