A replica-plating technique has been adopted for the isolation of mutagen-sensitive mutants of Chinese hamster V79 and CHO cell lines. After the mutagenic treatment (ENU) clones derived from these cell lines were replica plated into micro wells and replicas were treated with UV (254 nm), X-ray, MMC, EMC or MMS. Clonal cell lines which demonstrated mutagen sensitivity were retested by the determination of survival. Only one UV-sensitive line was obtained in 1500 clonal lines derived from CHO cells. This mutant appeared also sensitive to 4NQO and MMC. The sensitivity to UV and MMC was 2-3-fold enhanced, while the increase in sensitivity to 4NQO was 4-5-fold. In V79 cells 9 mutagen-sensitive lines were found after screening of 500 clonal lines; six of them showed increased sensitivity towards UV, two towards MMC, and one cell line was found to be X-ray sensitive. A considerable cross-sensitivity for the various agents was found among the isolated mutants. When a 2-fold increase is taken as a minimum to indicate mutagen sensitivity 6 mutants were sensitive to UV, 8 mutants were sensitive to MMC, 6 mutants were sensitive to 4NQO and 4 mutants were sensitive to X-rays. The difference in sensitivity to UV versus 4NQO makes it unlikely that 4NQO can be considered as a UV-mimetic agent. The sensitivity to MMC appears to fall into 2 classes: a class with moderate sensitivity (2-8-fold) and a class with high sensitivity (30-100-fold). The presence of similar classes is indicated for UV. Except for the two lines V-E5, V-B7 and the two lines V-H11, V-H4 all obtained mutants have a different spectrum of mutagen sensitivities which suggests that different genetic alterations underly these effects. The observed high frequency of mutagen-sensitive mutants in V79 cells, although unexpected and substantially higher than those published for CHO cells and L5178Y cells, can still be explained by the presence of functionally hemizygous loci.