The expression of intercellular adhesion molecule-1 (ICAM-1) by human cerebral endothelium was studied in primary cultures of human brain microvessel endothelial cells following treatment with bacterial lipopolysaccharide (LPS), tumor necrosis factor-alpha (TNF-alpha), interleukin-1 beta (IL-1 beta) and interferon-gamma (IFN-gamma). Surface expression of ICAM-1 was examined with the immunogold silver staining technique. Intact cerebral endothelial cells constitutively express low levels of ICAM-1. Stimulation with LPS and cytokines induces upregulation of ICAM-1 which is minimal with IFN-gamma and maximal with LPS or a combination of IFN-gamma and TNF-alpha. Upregulation of ICAM-1 expression is concentration- and time-dependent, is observed as early as 4 h following incubation and persists for up to 72 h in the continuous presence of LPS or cytokines. The ICAM-1 expression is not reversed by 3 days after removal of the LPS or cytokines. These findings may be relevant to the interactions between leukocytes and brain microvessel endothelial cells in inflammatory and demyelinating diseases of the CNS.