The relationship between the endogenous cytoplasmic levels of the enzymes superoxide dismutase and catalase and the inhibition of cell proliferation by radiation has been studied in 11 mouse cell lines. The resistance of these mouse cell lines to radiation was found to vary by over 25-fold. No correlation was found between the cytoplasmic level of CuZn-superoxide dismutase or catalase and the resistance to radiation as measured by extrapolation number (EN), quasi-threshold dose (Dq), or DO. None of the cell lines had detectable cytoplasmic Mn-superoxide dismutase. The apparent Ki of potassium cyanide for mouse CuZn-superoxide dismutase was determined (Ki = 6.5 mumol dm-3).