The etiology of rheumatoid arthritis (RA) remains unknown; however, recent studies have suggested a central role of the vascular endothelium in RA pathogenesis. The immune complex (IC)-mediated vasculitis is typical for RA. The studies reported herein were undertaken to determine 1) whether IC isolated from plasma of patients with RA are capable of inducing expression of ICAM-1/CD54 and Fas antigen/CD95 on the endothelial cell (EC) in vitro, 2) whether the capacity to induce expression of this phenotypic markers of EC activation is determined by the size or by the composition of the IC. The concentrations of IC were chosen to be within the range for IC levels in plasma. We have shown that all IC-containing samples (n = 8) significantly and in a dose-dependent manner increased level of ICAM-1 expression on the EC. In selected experiments EC were incubated with IC in the presence of complement. The presence of serum containing active complement components resulted in further up-regulation of ICAM-1 expression only in 4 of 8 cases, independently on the ability of IC to fix complement. Additionally, we have found that all IC samples significantly and in a dose-dependent manner induced the marked increase in CD95 expression on the EC. Furthermore, the levels of augmented expression of CD95 correlated with the levels of CD54 expression stimulated by the same IC-containing samples. We have demonstrated that these levels of stimulated expression of ICAM-1 as well as levels of Fas antigen expression negatively correlated with percentage amounts of IgM in total protein concentration of IC and directly correlated with IgG content in comparison to IgM in the structure of this IC. Our results show that IC stimulate ECs to express ICAM-1 and CD95, all of which have been implicated in the pathogenesis of rheumatic disease. The studies reported herein provide further information regarding to inflammatory potential of IC in RA.