Cell killing and the induction of mutation to thioguanine resistance (HGPRT enzyme deficiency) were measured after exposure of Ehrlich ascites tumor cells to 150-kV X rays and 241Am α particles. The curve describing the induction of mutations was almost linear after exposure to α particles (slope: 14.1 x 10-5 Gy-1) but upward bending after exposure to X rays, apparently reaching a final slope similar to that obtained after exposure to α particles. The number of mutants induced per viable cell by α particles at a given level of cell killing was similar to that induced by X rays. The RBE values obtained for cell killing and the induction of mutations are compared with each other, and the possible involvement of repair processes in determining the RBE is discussed.