IN 1953, Gray and co-workers1 demonstrated that Ehrlich ascites tumor cells showed increased sensitivity to x-ray irradiation when the cells were also exposed to high concentrations of oxygen. Subsequent laboratory studies2 and clinical trials3 have substantiated the evidence for potentiation of x-ray irradiation by high pressure oxygen (HPO, hyperbaric oxygen).
Because x-ray irradiation and the alkylating agents have clinical, cytological, and biochemical similarities in their effects,4,5 several investigators have tested the combined effect of high pressure oxygen and chemotherapy in the laboratory,4,6-8 and two groups have performed clinical trials.9,10 We are reporting a study of the individual and combined effects of nitrogen mustard and high pressure oxygen as suppressors of Ehrlich ascites tumor in mice. The parameters for response to treatment were survival and the total number of viable tumor cells. The latter was calculated as the product of the tumor cell count per