Certain bis(2,6-dioxopiperazine) derivatives, which include ICRF-187 [(+)-1,2-bis(3,5-dioxopiperazinyl-1-yl]propane; ADR-529) and its racemic compound ICRF 159 (Razoxane), have been investigated as antineoplastic agents. In addition, ICRF-187 is currently under intense study as an agent to ameliorate the cardiac toxicity of anthracycline therapy. These agents have recently been identified as inhibitors of topoisomerase II. We studied the effects of ICRF-187 and ICRF-159 on the progression of cultured epithelial cells through M phase. Beginning approximately 1.5 h after drug addition, chromosome condensation was significantly inhibited. Cells entered and progressed through M phase at near normal rates, but the lack of complete chromosome separation during anaphase resulted in catastrophic effects on normal chromosome distribution. Immunolabeling with Crest autoimmune sera, which recognizes centromere proteins, and with MPM-2 monoclonal antibody, which recognizes mitotic phosphoproteins, indicated that the centromeres of the chromosomes assembled a normal metaphase array in the presence of ICRF-187 and ICRF-159. Centromere separation in anaphase was initiated normally but was not completed because the chromatid arms failed to disengage from each other. Massive chromosome bridges were formed, and the chromatin mass became trapped in the cleavage furrow leading to its unequal distribution to the daughter cells. In many cases, all the chromatin was pushed into one of the two dividing cells. It is likely that previous studies, based on flow cytometry, indicating that bis(2,6-dioxypiperazine) derivatives cause an accumulation of cells with a 4N DNA content, reflect the incomplete segregation of chromosomes in mitosis rather than a block in G2 of the cell cycle as had been proposed.