We have succeeded in corroborating the enhancing effect of vitamin A, vitamin C, sitosterol and fucosterol on the fibrinolytic activity of endothelial cells. The assay system consisted of an in situ dissolution of a fibrin layer coated onto a culture dish, over which endothelial cells were grown in a culture medium containing 10% serum. The dissolution was enhanced by the addition of these vitamins and phytosterols to the culture medium.