Stable variants resistant to hypertonic stress have been obtained from V79 cells by one-step selection in media supplemented with graded concentrations of NaCl. Such variants retain a potential for resistance when isolated and propagated in isotonic media. On replating in graded NaCl, a family of dose-response curves is obtained, rising in level of resistance according to the degree of hypertonicity used to isolate the variants initially. In hybrids between variants and sensitive cells, phenotypic expression of resistance to hypertonic NaCl is recessive. Stable variants can also be isolated by one-step selection in media made hypertonic with D-mannitol. Clonal sublines selected with mannitol, as well as those obtained with NaCl, are resistant to both types of hypertonic media. Fluctuation tests in media supplemented with NaCl show that resistance arises spontaneously and at random, with measured rates of variation that depend on the concentration of NaCl used for selection. Treatment of sensitive cells with 5-azacytidine increases the frequency of resistant variants in assays with high levels of added NaCl but is less effective when selection is performed at lower concentrations. Exposure to ethyl methane sulfonate has little or no effect on variant frequency.