Co-culture experiments. Cells in lower compartments were selectively irradiated to induce paracrine proliferation of cells in upper compartments. Radiation (10Gy) of endothelial cells (HDMVECs) and fibroblast cells stimulated fibroblast proliferation (RT) in upper compartments. The administration of SU9518 significantly inhibited radiation-induced fibroblast proliferation. Likewise, irradiation of fibroblasts stimulated the proliferation of HDMVECs. The paracrine induction of endothelial cell proliferation was significantly inhibited by SU9518. The basal paracrine mitogenic stimulation of fibroblasts or endothelial cells in the upper compartment induced by co-culturing with non irradiated control cells in the lower compartment was also effectively inhibited by SU9518 (all SU9518 vs. control groups). Bars are mean ± SD. In all figures asterisks indicate degree of statistical significance: *significant, P < 0.05; **highly significant P < 0.01.