We have characterized selected populations of fibrosarcoma cells separated on density gradients of Renografin with respect to distribution in the cell cycle, clonogenic ability, and response to radiation. The latter two parameters have been observed to be related to the density of the cells, the denser populations being less clonogenic but more resistant to both low and high LET radiation. We have determined that these cells effectively repair potentially lethal damage if allowed to remain in situ following exposure to low LET radiation. This repair ability is not observed, however, following neutron irradiation. In addition, cells left in situ are relatively more sensitive to low doses of either low or high LET radiation. This difference in response is believed to be due to differences in the expression of damage by cells left in situ as compared to those assayed immediately. These data reflect the usefulness of cell separation methods in the study of solid tumour systems. Specifically, these methods reduce the heterogeneity found in the solid tumour by allowing for the isolation of subpopulations which then can be studied either individually or in relation to the entire tumour.