In order to test one of the possible radiomimetic qualities of alkylating agents, we have studied the influence of hyperbaric oxygen on the tumoricidal action of mechlorethamine (nitrogen mustard) in spontaneous mammary tumors of heterozygous, randomly bred Swiss-Webster mice. Since the initiation of this study, two reports have been published relating to it. In a similar test, Krementz and Knudson,6,7 using the Ehrlich ascites tumor, sarcoma 180 and sarcoma 37, demonstrated a significant decrease in cell counts and mortality in the Ehrlich ascites tumor, but they were unable to show any change in the sarcoma 180 or sarcoma 37. Back,3 working with sarcoma 180, leukemia 1210, and Gardiner lymphosarcoma, found no change in the mechlorethamine sensitivity in environments of 100% oxygen at up to four atmospheres absolute pressure. In both of these studies, transplanted tumors were used, in contrast to the spontaneous tumor used in the present report.