Stromal cell progenitors (fibroblastoid colony-forming unit; CFU-Fs) are representative of the progenitor cell population of the hemopoietic microenvironment in bone marrow (BM). Previous studies of the radiation dose-effect relationships for colony formotion have shown that canine CFU-Fs are relatively radioresistant as characterized by a DO value of about 2.4 Gy. In contrast, hemopoietic progenitors are particularly radiosensitive (Do values = 0.12-0.60 Gy). In the present study, the alkaline singlecell gel electrophoresis technique for the in situ quantitation of DNA strand breaks and alkalilabile sites was employed. Canine buy. coat cells from BM aspirates and cells harvested from CFU-F colonies or from mixed populations of adherent BM strornal cell (SC) layers were exposed to increasing doses of X-rays, embedded in agarose gel on slides, lysed with detergents, and placed in an electric field. DNA migrating from single cells in the gel was made visible as "comets" by ethidium bromide staining. Immediate DNA damage was much less in cultured stromal cells than in hemopoietic cells in BM aspirates. These results suggest that the observed differences in clonogenic survival could be partly due to differences in the type of the initial DNA damage between stromal cells and hemopoietic cells. o 1996