The level of radiosensitivity of neoplastic cells in vivo and of spheroids in vitro can be modified by controlling their rate of O2 consumption. An attempt was made to compare this rate in the melanotic and amelanotic lines of Bomirski hamster melanoma in vitro; these 2 lines distinctly differ in their reactivity to ionizing radiations. The measurements carried out by the use of a new ESR method revealed that pigmented and pigmentless cells consume O2 at significantly different rates. O2 utilization may contribute to the overall level of radioresistance of melanoma cells.