The induction of mutation by .alpha. particles emitted by Pu-238 (LET [linear energy transfer] at the cell surface 100 keV/.mu.m) and 250 kVp [kilovolt peak radiation] X-rays was measured utilizing the hypoxanthine-guanine phosphoribosyl transferase (HGPRT) locus in Chinese hamster ovary (CHO) cells. Dose-response relationships for mutagenicity and survival of CHO cells are presented. Linear plots of mutations to 6-thioguanine resistance describe cumulative curves which reveal nonthreshold responses for both radiations. Although neither of these curves is linear over the entire range of doses shown, the initial portions of both curves are linear with non-zero slopes. The linear slopes for .alpha.- and X-ray induced mutations are 4.79 .+-. 0.50 and 1.30 .+-. 0.23 .times. 10-7 mutations/viable cell per rad, respectively. Plots of mutation frequency against survival show that for a given level of survival, .alpha.-irradiation is relatively less mutagenic than is X-irradiation.