We have previously presented evidence that a transient block to DNA replication induces an aberrant form of DNA synthesis. The most feasible explanation for this data is that the block to DNA replication results in some segments of the chromosomal DNA being replicated more than once in a single cell cycle. This form of aberrant DNA synthesis was demonstrated to occur following direct inhibition of DNA replication by 1-beta-D-arabinofuranosylcytosine or 9-beta-D-arabinofuranosyladenine or after indirect inhibition with cycloheximide. We have proposed mechanisms whereby this phenomenon could induce chromosome damage and cell death. In this paper, we present data on the relationship between this aberrant form of DNA replication and the loss of cell viability. Using Chinese hamster ovary CHO-K1 cells growing as monolayer cultures, we have simultaneously monitored the loss of cell viability as measured by colony formation and the relative extent of this aberrant DNA replication induced by 2-hr pulses of a series of concentrations of inhibitors of DNA replication. We have found that, with either direct inhibition of DNA replication with 1 beta-D-arabinofuranosylcytosine or with indirect inhibition, with cycloheximide, pulses of inhibitor administered to Chinese hamster ovary cells at increasing of this aberrant DNA replication which paralleled the increase in cell killing.