Image analysis was used to study the repair process of a circular mechanical lesion of confluent human endothelial cells in culture after irradiation (10 Gy) prior to wounding. Coverage of denuded areas 48 and 96 h after injury of endothelial cells was identical in control and irradiated cultures, although the labeling index was lowered by 80 to 95% in irradiated cultures. The cell density of non damaged irradiated areas was decreased by 50%. When cultures were submitted to increasing doses of radiation (5.0-30 Gy), the labeling index of the cells diminished rapidly between 0 and 5.0 Gy and reached a plateau at 10 Gy. The decrease in cell density (50% of control at 96 h) was identical at each dose of radiation. Thus cell migration alone could be sufficient for the repair of the lesion, while cell proliferation would mainly maintain the original cell density. The addition of heparin to the culture medium slowed down cell migration and proliferation, but the speed of repair was identical in irradiated and non-irradiated cultures. Acidic fibroblast growth factor plus heparin accelerated equally the repair process whether the cultures were irradiated or not. In irradiated cultures the presence of acidic fibroblast growth factor and heparin maintained cell density in confluent areas at a level similar to that in non-irradiated damaged control cultures without addition of mitogens. Thus heparin and acidic fibroblast growth factor play a role in cell proliferation, in the maintenance of the cell monolayer integrity and in restoring a continuous layer by rapid cell migration and elongation after irradiation.