Two subpopulations of tumor cells (clones A and D) obtained from a human colon adenocarcinoma were examined for their sensitivities to x-irradiation as unfed, early plateau phase cultures. Both the single dose survival curves and the kinetics of potentially lethal damage recovery (PLDR) were determined for the two tumor lines. Also, possible modification of PLDR by N,N-dimethylformamide (DMF), which has previously been shown to enhance the radiosensitivity of exponentially growing tumor cells, was investigated by adding DMF (0.8% v/v) to plateau phase cultures immediately after irradiation, and determining effects on the extent of PLDR. For non-DMF treated cells, the survival curve parameters of the diploid (clone D) and aneuploid (clone A) lines were very similar. The single-hit, multitarget values for n, Do (Gy), and Dq (Gy) were: 7.9, 0.82, and 1.70 for clone D; and 10.6, 0.83, and 1.96 for clone A. Using initial survival levels of 3.5% (clone D) or 5.5% (clone A) to investigate PLDR, it was found that the increase in survival (surviving fraction ratio or SFR) for clone D was 2.2, while the SFR for clone A was 1.6. DMF did not change either the kinetics or extent of PLDR in these two tumor lines when added to cultures immediately after irradiation. Our results indicate that significant heterogeneity in PLDR exists between these closely related tumor subpopulations.