The induction of neoplastically transformed cells has been measured with single and fractionated doses of 50-kV X rays and fission-spectrum neutrons from the JANUS reactor at the Argonne National Laboratory. Combining these results with parallel measurements of cell survival permits an analysis of the possible roles of repair processes in the expression of sublethal and subtransformation damage. It is concluded that sub-transformation damage due to X rays is repaired although probably more slowly than sublethal X-ray damage. Much less, if any, repair occurs between neutron doses for periods up to 24 hr.

The presence of 0.75 μM N-ethylmaleimide (NEM) during irradiation of synchronous HeLa cells at different stages of their cycle enhances cell killing (i.e., sensitizes) in early G 1 and late S, but does not affect mitotic cells or cells at the G 1 / 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 G 1 . 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 G 1 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 presence of 0.75 μM NEM produced almost no variation in survival through "the cycle," the inhibited cells showing a sensitivity greater than that normally observed in mitotic cells. The results presented are consistent with the idea that 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.