The radiobiological properties of a cyclotron-produced 43-MeV (p----Be) fast-neutron beam relative to gamma rays have been investigated using Chinese hamster V79 cells in culture. As expected, the relative biological effectiveness (RBE) of this neutron beam for cell killing was shown to increase as dose decreased, and the effectiveness per unit dose was slightly less compared to a 25-MeV (d----Be) neutron beam. By tracing single cells that formed microcolonies after irradiation, we found cell proliferation kinetics to be retarded to a greater extent by fast neutrons than by gamma irradiation. Following either neutron or gamma irradiation, a fraction of the irradiated cells failed to divide in the first postirradiation division and another fraction could produce as many as four generations of progeny before proliferation stopped. The properties of these cells presumed to be destined for death suggest that more than one mechanism and/or multistep process underlies the radiation-induced proliferative death. The fast-neutron beam was also found to be more effective quantitatively than gamma rays in producing DNA double-strand breaks (DSBs, measured by nondenaturing filter elution), and G1-phase chromosome fragments (measured by the premature chromosome condensation technique). However, the reverse was observed for DNA single-strand breaks (SSBs, measured by alkaline filter elution or hydroxylapatite uncoiling). Interestingly, both fast neutrons and gamma rays produced a large component of SSBs and DSBs with a fast-rejoining time constant of about 2-5 min, which appears to be independent of dose. The latter results could not resolve the possibility of lengthening the repair-time constant by increasing radiation dose within the range that is reflected by the shoulder of the survival curve, and consequently did not support the idea of repair saturation as a mechanism for the presence of the shoulder. The RBE for the hypoxanthine phosphoribosyl transferase mutation frequency per survivor at the 10% survival level was estimated to be 2.5, a value that is comparable to the RBE (2.1) for cell killing at the same survival level. Although most of the above-mentioned findings are compatible qualitatively with the relatively high-LET (linear energy transfer) nature associated with the fast-neutron beam, the significance of the action attributable to the mixture of LET could not be delineated in these experiments. Further, the biological significance of DSBs and chromosome aberration and the molecular mechanisms responsible for the repair and expression of these damaging processes remain to be elucidated.