Protection experiments were carried out with washed suspensions of Escherichia coli B/r suspended in buffer, exposed to 0.1 [image] L-cysteine at 22-24[degree]C, and irradiated with 250-kv X-rays at 0[degree]C. Suspensions were in equilibrium with 0, 5, 20, or 100% oxygen at the time of irradiation. L-cysteine afforded significant protection (dose factor. 2) under all conditions. The mode of action of cysteine therefore includes one reaction independent of oxygen. The independence is both biologic and radiologic, i.e., oxygen is not needed for cysteine to reach the sensitive locus prior to irradiation, nor is it needed at the time of irradiation. It was also shown in tests at pH 7.4-7.8 that the ethyl ester of L-cysteine and D-cysteine protect under anoxic conditions; D-cysteine, however, was significantly less potent than L-cysteine at pH 5.