The morphological behaviour of leucocytes* leaving the blood has been studied for more than a hundred years and can be described in some detail, but the mechanisms involved are for the most part still unclear. Following an inflammatory stimulus, dy- namic changes occur in small blood vessels near the inflamed site and are followed by margination of leucocytes and by their adherence to the endovascular surface of endo- out of the vessel in the gaps between endothelial thelial cells. The leucocytes migrate cells and move towards the noxious stimulus, directed presumably by chemotactic gra- dients. Having reached the gradient source, they may engage in phagocytosis, killing and digestion of micro-organisms or of damaged tissue. In this review, we deal with the adhesive properties of leucocytes, which have US1,l;ll- ly been measured in vitro, and with their possible relevance to the leucocyte-endo- We also deal with the locomotion of leucocytes, its machinery thelial cell interaction.