The findings of Hill et al. (1984) on the greatly enhanced transformation frequencies at very low dose rates of fission neutrons induced us to perform an analogous study with alpha-particles at comparable dose rates. Transformation frequencies were determined with gamma-rays at high dose rate (0.5 Gy/min), and with alpha-particles at high (0.2 Gy/min) and at low dose rates (0.83-2.5 mGy/min) in the C3H 10T1/2 cell system. alpha-particles were substantially more effective than gamma-rays, both for cell inactivation and for neoplastic transformation at high and low dose rates. The relative biological effectiveness (RBE) for cell inactivation and for neoplastic transformation was of similar magnitude, and ranged from about 3 at an alpha-particle dose of 2 Gy to values of the order of 10 at 0.25 Gy. In contrast to the experiments of Hill et al. (1984) with fission neutrons, no increased transformation frequencies were observed when the alpha-particle dose was protracted over several hours.