The effects of in vitro irradiation on proliferation and hematopoietic supportive functions of stromal cells were studied. To assess the effects of radiation on stromal cells proliferation, marrow cells were exposed to a single dose of gamma radiation. The 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assay showed that stromal cell proliferation was significantly suppressed after radiation in a dose-dependent manner. Stromal layers obtained from irradiated marrow cells failed to establish adherent layers after 6, 8, or 10 Gy of radiation. To assess the functions of stromal cells that survived radiation, stromal layers derived from irradiated marrow cells were cocultured with freshly isolated autologous hematopoietic cells and assayed for their capacity to support prolonged granulocyte-macrophage progenitors (CFU-GM) production. Stromal layers derived from 2-Gy-irradiated marrow cells resulted in similar CFU-GM production as control cells, while stromal layers derived from 4- to 10-Gy-irradiated marrow cells significantly decreased CFU-GM production. To study the influence of radiation on hematopoietic supportive capacity in established stromal layers, stromal layers generated from non-irradiated marrow cells were irradiated and cocultured with freshly isolated autologous hematopoietic cells. Established stromal layers irradiated up to 10 Gy sustained prolonged CFU-GM production, suggesting that hematopoietic stromal supportive functions remained intact at this dose of radiation. In conclusion, our results indicated that proliferation of stromal cells and bone-marrow stromal layer formation from stromal cells are sensitive to radiation in vitro, while established bone-marrow stromal layer originating from stromal cells is relatively resistant to radiation. Data generated may have implications in future bone-marrow transplantation research.