The presence of 0.75 .mu.M N-ethylmaleimide (NEM) during irradiation of synchronous HeLa cells at different stages of their cycle enhances cell killing (i.e., sensitizes) in early G1 and late S, but does not affect mitotic cells or cells at the G1/S border. The effect is primarily on the shoulder of the survival curve, rather than its slope. NEM given immediately before or after irradiation is as effective as during irradiation. When given as a function of time after exposure the effect decreases in magnitude, cells becoming insensitive to NEM more rapidly in S than in G1. The drug is also effective if administered prior to irradiation, the effect generally decreasing the longer the interval between treatment and irradiation. The addition of 1.0 mM hydroxyurea (HU) to synchronous HeLa cells in G1 showed that survival changes occur as the cells proceed through the cell cycle even in the absence of DNA synthesis. The changes observed are qualitatively the same to those observed previously in Chinese hamster cells. The irradiation of HU-inhibited cells in the response of 0.75 .mu.M produced almost no variation in survival through the cycle, the inhibited cells showing a sensitivity greater than that normally observed in mitotic cells. NEM inhibits repair processes in the cell most likely due to its binding to a critical SH-containing enzyme(s) concerned with the repair of radiation damage.