Inflammatory stimuli enhance the adherence properties of human umbilical vein endothelial cells (HUVEC) for neutrophils (PMN). This is mediated in part by the up-regulation on HUVEC of Intercellular Adhesion Molecule 1 (ICAM 1). Phorbol esters, which activate protein kinase c (PKC), have also been reported to enhance the adherence properties of HUVEC for PMN. We investigated the effect of agents which activate PKC on the expression of ICAM 1 by HUVEC. Both phorbol myristate acetate (PMA) and Mezerein, a non-phorbol which also stimulates PKC, enhanced both the expression of ICAM 1 on HUVEC and the adherence of HUVEC for PMN. The PKC inhibitors staurosporine and H-7 prevented both PMA and Mezerein-induced stimulation of HUVEC expression of ICAM 1 and adherence for PMN. We conclude that activation of PKC in HUVEC is associated with increased expression of ICAM 1 on HUVEC. PKC-mediated up-regulation of ICAM 1 may be responsible, in part, for the promotion of endothelial cell adherence properties toward PMN.