When cells of Escherichia coli B growing in a glucose-synthetic medium were treated with mitomycin C, the effects produced by the antibiotic varied, depending on the concentration. When the concentration was reduced to less than 0.1 mug/ml, the action of the antibiotic was bacteriostatic; cell elongation resulted, but no effect on the synthesis of cellular macromolecules was apparent. At higher levels (more than 5 mug/ml), mitomycin C was highly bactericidal and inhibited deoxyribonucleic acid synthesis almost completely. The exposure of growing cells to a bactericidal level of mitomycin C resulted also in a delayed inhibition of the synthesis of ribonucleic acid (RNA) and protein. The capacity of the treated cells to synthesize beta-galactosidase inducibly in a medium free from a carbon source remained constant for the first 30 min and then was destroyed progressively with time. Prolonged incubation with the bactericidal level of mitomycin C caused a degradation of cellular nucleic acids, particularly RNA. The degraded nucleic acid components were eventually released into the medium.