OBJECTIVE The objective is to determine the contributions of electrostatic charge, endothelial cell adhesion glycoproteins (P- and E-selectins), and histamine to lactoferrin-induced leukocyte adhesion in postcapillary venules. METHODS Rat mesentery was prepared for intravital microscopic observation. Erythrocyte velocity, vessel diameter, leukocyte rolling velocity, number of adherent and emigrated leukocytes, and velocity, flux, and number of rolling leukocytes were monitored in mesenteric venules (25-35 microns initial diameter). After control measurements were obtained for all parameters, lactoferrin, other cationic proteins (histone or alpha-chymotrypsinogen A), or transferrin (an anionic iron-binding protein) were infused into the superior mesenteric artery, with repeat measurements taken 20 min into the infusion period. In other lactoferrin experiments, animals were treated with either a monoclonal antibody (MAb) directed against P- or E-selectin, an H1- or H2-histamine receptor antagonist, or diamine oxidase (histaminase). RESULTS Increased numbers of rolling and adherent leukocytes were observed during infusion of lactoferrin, histone, or alpha-chymotrypsinogen A but not with transferrin. The leukocyte-endothelial cell adhesion (LECA) elicited by lactoferrin was substantially greater than that induced by histone and alpha-chymotrypsinogen A. The P-selectin MAb completely prevented lactoferrin-induced LECA, whereas the E-selectin MAb had no effect. Diamine oxidase and the H1- (but not the H2-) receptor antagonist were also effective in attenuating lactoferrin-induced LECA. CONCLUSIONS These results indicate that lactoferrin-induced LECA results from histamine-mediated expression of P-selectin on venular endothelial cells. The cationic nature of lactoferrin accounts for only a small part of its proadhesive actions.