Cells of the desmid C. moniliferum are more resistant to killing by radiation if they are previously exposed to relatively small doses of ionizing radiations (14 MeV neutrons, 290 kVp X-rays, or 10 MeV electrons) or 254 nm UV light. The effectiveness of the resistance-inducing dose reaches a plateau with increasing dose and is insensitive to dose rate or the presence of O2. Neutrons (14 MeV) are about 2.4 times more effective than electrons or X-rays in inducing resistance. The damage responsible for resistance induction appears to be different from that leading to cell death or mitotic delay. The hypothesis that this increased resistance is due to the induction of enzyme which enhances the repair of sublethal damage is supported by the effects of low temperature or cycloheximide.