Environmental modification of the intrinsic ionizing radiation sensitivity of two tumor cell subpopulations obtained from a single heterogeneous human colon adenocarcinoma was studied. The two subpopulations (termed clones A and D) were grown as monolayers to produce plateau phase cultures. Two different plateau phase growth states were produced by either changing the culture medium daily until maximum density was achieved ("fed" cultures), or by not changing the original medium for the entire duration of growth ("unfed" cultures). Cells were irradiated with graded single doses of X rays to determine survival responses. Using conventional single-hit, multi-target terminology, the extrapolation numbers (n), mean lethal doses (D0, Gy), and quasi-threshold doses (Dq, Gy) for fed and unfed cultures of clone A cells were 23.7, 0.80, and 2.54 (fed); and 9.65, 0.85, and 1.93 (unfed). For fed and unfed cultures of clone D cells these values were: 13.0, 0.76, and 1.94 (fed); and 23.8, 0.73, and 2.30 (unfed). No significant difference was found between the radiation responses of these two tumor subpopulations in the plateau phase of growth, either as fed or unfed cultures. The D0 value for clone D, but not for clone A, was significantly less than the D0 value for cells in exponential growth, indicating that the change from exponential growth to a plateau growth state is accompanied by a change in the intrinsic sensitivity of clone D tumor cells. After establishment of the single dose response curves, we then investigated the ability of clone A and D cells to recover from radiation injury (potentially lethal damage recovery, PLDR) by irradiating plateau phase cultures with a single radiation dose sufficient to reduce survival to about 1%, and then subculturing the cells into fresh medium at times varying from 0-24 hours postirradiation. We found that the maximum increase in survival was seen at 12-24 hours postirradiation, and that the ratio of this maximum survival to the cell survival when subculturing was done immediately after irradiation was 2.7 (fed) and 2.5 (unfed) for clone A cells, and 3.6 (fed) and 3.0 (unfed) for clone D cells. Therefore, whereas clone D cells have a greater ability to express PLDR (by a factor of about 1.3) than clone A cells, the modification of this recovery by the environment was identical, with survival in the fed state being greater than in the unfed state by a factor of 1.2 and 1.1 for clone D and clone A tumor subpopulations, respectively.