The effects of selenite on DNA integrity, cell viability, and long-term proliferative potential of mouse leukemic L1210 cells were examined in this study. Selenite treatment resulted in concentration-dependent increases in DNA single-strand breaks and double-strand breaks, as detected by a modified filter elution assay. A time-course experiment showed that DNA single-strand breaks preceded DNA double-strand breaks. Agarose gel electrophoresis of DNA extracted from selenite-treated cells displayed a nucleosomal fragmentation pattern that is characteristic of apoptotic cell death. The involvement of a Ca2+,Mg(2+)-dependent endonuclease responsible for DNA double-strand fragmentation was implied by the observation that two inhibitors of endonuclease activity, i.e. aurintricarboxylic acid and zinc, blocked selenite-induced DNA double-strand breaks. These inhibitors also prevented selenite-induced cell death as defined by loss of ability to exclude trypan blue dye. Selenite treatment severely impaired the colony-forming ability of cells capable of trypan blue exclusion. The induction of DNA strand breaks and commitment to apoptosis may explain the selenite-mediated growth inhibition and loss of long-term proliferative potential.