The effect of purified fibrin(ogen) degradation product fragment D on cultured bovine aortic endothelial cells has been investigated. Binding to the cells was studied using purified radiolabelled fragment D and assessment of its recently postulated deleterious action was made by three different approaches: release of 51Cr from prelabelled cell layers, uptake of 3H-adenine by the cultivated cells as well as determination of residual cell number after exposure to various concentrations of fragment D were carried out. None of these methods applied was able to detect any harmful effect of the fragment on endothelial cell integrity and function in vitro. Neither could a specific interaction of the protein with the cultured cells be established. Even huge amounts of fragment D (1 mg/ml) exposed to the cells over 20 h did not reveal any alterations in the typical monolayer morphology. Thus, fragment D, which is generated in large amounts during systemic fibrinolytic treatment, does not exert any perturbation on cultured vascular endothelial cells as had previously been proposed.