The pattern of early cell movement after an experimental 'wound' and the organization of actin in stationary and moving cultured endothelial cells have been studied by means of: time-lapse photography; indirect immunofluorescence using anti-actin antibodies with and without pretreatment with the actin destabilizing factor present in human plasma; and differential centrifugation and densitometric analysis of stained sodium dodecylsulphate/polyacrylamide gels in order to evaluate the total and relative amounts of G and F-actin. Up to 5 h after a single scratch, movement consists of a coordinate spreading and translocation of a band of about 10 cells from the wound edge. Compared to stationary cells, moving endothelial cells show: no significant changes in the intensity and distribution of immunofluorescent staining with anti-actin antibodies, but an increased sensitivity of cytoplasmic actin, including stress fibres, to the actin-destabilizing factor purified from human plasma; and no significant change in the total amount of actin, but a decreased relative amount of F-actin and a corresponding increased relative amount of G-actin. We conclude that endothelial cell movement in vitro is accompanied by a rapid change in the state of actin organization characterized by an overall decrease in cytoplasmic F-actin.