The mechanism by which the carcinogen 4-nitroquinoline-1-oxide (4-NQO) kills mammalian cells is unclear; however, damage to DNA is presumed to be involved. We examined the relationship between the kinetics of cell death and alteration of cell-cycle compartmentalization after exposure of T5-1 human lymphoblastoid cells to 4-NQO (50 to 500 ng/ml) to establish whether cytotoxicity was related to the perturbation of DNA replication. Dose-dependent reductions in cell proliferation and cell viability were present from 1 day after treatment. Maximal reductions in viability were observed 2 days after exposure. Concentrations of 4-NQO of up to 175 ng/ml did not affect cell-cycle compartmentalization, but 250 ng 4-NQO/ml caused a transient accumulation of cells in S phase after 1 day. Only after 500 ng 4-NQO/ml was a marked and prolonged S-phase block observed from 1 day onwards. The lack of a strong correlation between S-phase block and cell death after exposure to 4-NQO suggests that responses to DNA damage in addition to perturbation of DNA replication entrain 4-NQO-induced cell lethality.