Stable hydrogen peroxide (H2O2)-resistant variants of the Chinese hamster ovary HA-1 line have been isolated by culturing cells in progressively increasing concentrations of H2O2 (greater than 200 days, in 50-800 microM H2O2). Increases in catalase activity in these variant cell lines were shown to correlate with increased H2O2 resistance. Stable (greater than 240 days) H2O2-resistant cell lines, seven quasidiploid (21-22 chromosomes/cell) and six quasitetraploid (40-44 chromosomes/cell) were clonally isolated from the 800 microM adapted H2O2-resistant variants which were heterogeneous with respect to ploidy. The H2O2 dose-modifying factors (DMFs) were 3, 5, 8, 13, 15, 26, and 27 for the seven quasidiploid cell lines, and 21, 32, 38, 40, 42, and 49 for the six quasitetraploid cell lines. The mean DMF was 14 +/- 10 for the former and 37 +/- 10 for the latter. Our data show that on the average the quasitetraploid cell lines were significantly more resistant to H2O2-mediated cell killing than the quasidiploid cell lines derived from the same mixed population of 800 microM H2O2-adapted cells. When catalase activities (k units/cell) of the HA-1 cells and three of the clonally derived cell lines (two quasidiploid and one quasitetraploid) were determined and plotted vs. H2O2-DMF, a positive linear correlation was obtained (correlation coefficient = 0.99). This result was further confirmed when immunoreactive catalase protein/cell was detected by Western blots. Our data show that chronic exposure of cells to H2O2 stress (800 microM) was accompanied by increases in quasitetraploid cells within the population. Quasitetraploid cell lines derived from this population demonstrated increased stable H2O2-resistance which may be related to stable increases in the expression of catalase.