The responses of unfed plateau-phase cultures of two clonal subpopulations of cells (clones A and D) from a human colon adenocarcinoma (DLD-1) to X-irradiation were examined in detail either as control cultures or after growth in medium containing the differentiating agent N,N-dimethylformamide (DMF, 0.8%, three passages). Specifically, the cultures were studied with regard to their ability to express both potentially lethal and sublethal damage recovery (PLDR and SLDR, respectively). In PLDR studies on control cells, clone D expressed more PLDR than clone A, although recovery half-times were the same. DMF treatment increased the expression of PLDR in both cell lines and decreased the half-times for recovery. When recovery from sublethal radiation injury was assessed, the rate and extent of SLDR in non-DMF-treated clone A and D cells were identical. In contrast to the PLDR results, DMF treatment had no significant effect on SLDR in either cell line. These studies show that, while DMF treatment of human colon tumor cells increases cell killing in the clinically relevant, low-dose ("shoulder") region of the X-ray survival curve, this increase in cytotoxicity is not due to an inhibition of the repair of sublethal damage.