Imidazoacridinone C-1311 is a DNA-targeting antitumor intercalator/alkylator currently undergoing Phase II clinical trials. Here, we elucidated the sequence of death responses to C-1311 in human leukemia MOLT4 cells using drug concentration (30 nM) that causes near complete cell growth inhibition at 48 h. Early (6-12 h) responses included transient accumulation of cells at the G2/M border followed by also transient rise in several mitotic markers. Mitotic attempts were largely abnormal, resulting in numerous multinucleated cells (peaking at 24-39 h and declining markedly at later times). These events, indicative of mitotic catastrophe, were not associated with immediate cell death. The fraction of necrotic cells did not exceed 3%. Also, the classical manifestations of apoptosis were marginal at 24 h and their progression clearly followed the decline in the fraction of mitotic and multinucleated cells. Quantification of several apoptotic markers (including phosphatidylserine externalization, apoptotic DNA breaks, mitochondrial dysfunction, caspase activation, and cell membrane integrity) showed a considerable progression and the shift from early to late apoptosis at later times. At 72 h, >80% of cells were apoptotic. Collectively, these findings show that C-1311-induced mitotic catastrophe is not the ultimate death event but rather a step precipitating delayed, albeit massive, apoptotic responses.