APO-1 is a 48kDa transmembrane glycoprotein and belongs to the NGF/TNF receptor family of surface molecules. Cross-linking of APO-1 induces apoptotic cell death in sensitive cells. Here we show that APO-1 is an activation molecule on B cells. It could be induced/enhanced on dense and buoyant tonsillar B cells, respectively, through surface immunoglobulin cross-linking in combination with interleukin-2 or by interferon-gamma together with tumor necrosis factor-alpha. These conditions also increased the amount of intercellular adhesion molecule-1 (ICAM-1; CD54) on these cells. Epstein-Barr virus transformants of peripheral B cells co-expressed APO-1 and CD54 at very high levels. Immunohistologically, APO-1 was detectable at low levels in a subpopulation of follicular center B blasts and, at higher levels, in sinusoidal B cells. APO-1 was undetectable in follicular mantle B cells and plasma cells. Neoplastic B cell essentially mimicked their reactive counterpart with regard to APO-1 and CD54 expression.