The expression of intercellular adhesion molecule-1 (ICAM-1) on peripheral blood mononuclear cells (PBMC) from patients with systemic lupus erythematosus was observed by flow cytometry. ICAM-1 expression on PBMC was increased in the patients with active disease, and was increased especially on CD5- or CD19+ lymphocytes and monocytes. Furthermore, when B cells were fractionated by Percoll-density gradients, ICAM-1 was more expressed on the low- and the intermediate-density B cells, which are thought to contain activated B cells, than on the high-density B cells, which are thought to contain resting B cells. This might be due to the polyclonal B-cell activation occurring in vivo. Together with the function of B cells and monocytes as antigen presenting cells, this enhanced expression of ICAM-1 on these cells might be associated with the mechanism to maintain the pathophysiology of this disease.