Using cells from our tissue culture of human melanoma cell line Na 11, we transplanted 1 X 10)6) tumor cells sc into athymic nude mice. Tumors appeared after a latent period of 4-10 days; when they reached a mean volume of 100 mm3 we irradiated them with various doses of X-rays. Some tumors were irradiated while the mice were still alive; others were treated 10 minutes after the animals had been asphyxiated with nitrogen. All irradiation was done in the presence of oxygen. These tumors were excised, and cell suspensions were prepared; the cells formed colonies with a mean plating efficiency of 29%. In another series of experiments, we irradiated tumor cells in vitro 2 hours after excision, when most cells were fixed and presumably oxygenated. We then calculated survival curves for the tumor cells irradiated under these three conditions and found an average anoxic cell fraction of 85%, which was much higher than that reported in many other tumor systems. We explored several possible explanations for this phenomenon.