Mo3e is a protease-sensitive membrane antigen (p75,50) selectively expressed by human monocytic cells (monocytes and U-937 cells) stimulated in vitro by exposure to a variety of activating factors, including phorbol diester compounds, bacterial lipopolysaccharide (LPS), and muramyl dipeptide (MDP) (R. F. Todd et al., J. Immunol. 135, 3869, 1985). Here we report that primary and multiply-passaged cultures of HUVEC also express the Mo3e determinant after stimulation by phorbol myristate acetate (PMA) and related inducers of protein kinase C. As measured in a radioimmunoassay of anti-Mo3e antibody binding to monolayer cultures of HUVEC, unstimulated cells bore little if any Mo3e. After culture for 4-120 hr in medium containing PMA, 4β-phorbol dibutyrate, 4β-phorbol didecanoate, or mezerein (each at a conentration of 81 nM), or 1-oleoyl-2-acetoyl-sn-3-glycerol (1 mM), HUVEC were found to selectively express the Mo3e determinant. The magnitude of expression was dependent upon the concentration of the stimulus, maximal by 24 hr, and inhibited by cycloheximide. The combination of PMA and the calcium ionophore, ionomycin, had an additive or synergistic effect on HUVEC Mo3e expression. The biologically inactive phorbol compounds 4β-phorbol and 4α-phorbol didecanoate failed to stimulate Mo3e expression. Also inactive as inducers of HUVEC Mo3e expression were crude lymphokine and monokine supernatants, recombinant human lymphokines (interferon-γ and interleukin-2), recombinant human monokines (interleukin-1 and tumor necrosis factor), bacterial cell wall products including LPS and MDP, pharmacologic agents that increase intracellular cyclic adenosine monophosphate (prostaglandin E2, cholera toxin, theophylline, isoproterenol and isobutylmethylxanthine), lectins (Con A and PHA), and heparin. These results indicate that Mo3e is an inducible plasma membrane antigen of not only mononuclear phagocytes but also cultured HUVEC.