Microcystin-LR is a cyanobacterial toxin possessing a potent tumor-promoting activity mediated through inhibition of protein phosphatases PP1 and PP2A. Because these enzymes are involved in fundamental cell processes, we decided to examine the influence of microcystin-LR on cell cycle progression, onset of anaphase, segregation of chromosomes by the mitotic spindle, and apoptosis in Chinese hamster ovary (CHO-K1) cells. Cells were incubated with 25, 50, and 100 microM of pure microcystin-LR and a cyanobacterial extract for 14, 18, and 22 h. Giemsa staining of cells treated with these toxins revealed a dose- and time-dependent increase of mitotic indices, accumulation of abnormal G(2)/M figures with hypercondensed chromosomes, abnormal anaphases with defective chromosome separation, and polyploid cells. Because spindle checkpoint is a fundamental regulatory mechanism that assures the onset of anaphase and subsequent exit from mitosis, we examined the spindle organization in microcystin-treated cells. The majority of the mitotic cells showed monopolar and multipolar mitotic spindles (multiple asters). Microtubule bundles were present in interphase cells. Our results indicate that microcystin-LR induces apoptosis and necrosis in a dose- and time-dependent manner and that the frequency of dead cells cells is positively correlated with the frequency of polyploid cells.