When exposed to fast electrons at high dose rate, all cells of Closterium moniliferum survive (form colonies) up to a threshold dose of about 10 krad. If previously exposed to a dose between 0.14 and 10 krad they can survive higher threshold doses. We call this induced resistance and measure its extent by the ratio of doses to give the same killing effect with and without a previous exposure. We propose that induced resistance is due to the induction, by the first dose, of a repair mechanism which may be an extension of that which repairs sublethal damage. Resistance was optimally induced by doses between 1 and 5 krad, and developed rapidly with time after the first dose, being clearly apparent 1 hr after 2 krad and 3 to 4 hr after 19 krad. It developed slowly or not at all in the cold, suggesting that it is a metabolic process. (auth)