With confluent cultures of the C3H10T1/2 mammalian cell line, we have investigated the effects of heavy-ion radiation on neoplastic cell transformation. Our quantitative data obtained with high-energy carbon, neon, silicon, argon, iron, and uranium particles show that RBE is both dose- and LET-dependent for malignant cell transformation. RBE is higher at lower doses. There is an increase of RBE with LET, up to about 100-200 keV/micron, and a decrease of RBE with beams of higher LET values. Transformation lesions induced by heavy particles with LET values greater than 100 keV/micron may not be repairable in nonproliferating cells. RBE for slow and nonproliferating cells may be much higher than for actively growing cells.