Adhesion of neutrophils, lymphocytes and promyelocytic HL60 cells was compared in a flow-based model in which a monolayer of activated platelets formed the adhesive substrate. Each type of leucocyte formed P-selectin-mediated rolling attachments on the platelet surface under physiologically relevant flow conditions. Lymphocytes adhered less, and HL60 in similar numbers, compared to neutrophils, whereas the lymphocytes and HL60 cells rolled much more rapidly. Sulphated, sialylated saccharide(s) were implicated as ligand(s) for P-selectin for all leucocytes, but L-selectin (borne by neutrophils and lymphocytes, but not HL60 cells) appears to be a major presenter of ligand for neutrophils alone. T cells enriched from peripheral blood lymphocytes adhered in greater numbers than B cells. Differentiation of HL60 cells to neutrophil-like cells (induced by DMSO) caused cell volume to decrease and surface expression of integrin adhesion molecules to increase, but only a small percentage of cells were converted to an L-selectin-bearing phenotype. Differentiated cells showed evidence of stabilization of adhesion with increasing stress and a marked reduction in rolling velocity. These studies indicate that cell differentiation may be accompanied by alteration of adhesive behaviour, resulting from changes in physical characteristics as well as surface properties. Moreover, results suggest that P-selectin could promote lymphocyte attachment to endothelium in acute inflammatory conditions and possibly mediate lymphocyte-platelet interaction during thrombosis.