Cytogenetic effects of O6-ethylguanine (O6EtG) were investigated in V79 Chinese hamster cells and human lymphocytes. The frequency of micronuclei and of polyploid cells was increased in a dose-dependent manner in V79 cells. Micronuclei were larger than those induced by the alkylating agent diethylsulfate, suggesting that they originated from chromosome loss rather than from chromosome breakage. Chromosome displacement was also observed and further indicated damage to the spindle. In human lymphocytes, both aneuploidy and polyploidy were induced. O6EtG did not induce either micronuclei or polyploidy in a V79 derivative, deficient in hypoxanthine-guanine phosphoribosyl transferase, suggesting that conversion of the base to a nucleotide is necessary. Preliminary data indicated that 7-ethylguanine was also able to induce aneuploidy and polyploidy in lymphocytes, although it was barely active in V79 cells. The hypothesis proposed is that alkylated nucleotides may behave as spindle poisons. As a consequence, treatment of mammalian cells with alkylating agents may induce numerical chromosome aberrations via alkylation of guanine nucleotide pools, a pathway which has not so far been proposed.