APO-1 is a 48-Kd transmembrane glycoprotein identical to the Fas antigen and belongs to the nerve growth factor (NGF)/tumor necrosis factor (TNF) receptor family of surface molecules. Cross-linking of APO- 1 induces apoptotic cell death in sensitive cells. We show here that APO-1 is an activation molecule on B cells. It was induced/enhanced on dense and buoyant tonsillar B cells, respectively, through surface Ig 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 (CD54) on these cells. Epstein-Barr virus transformants of peripheral B cells coexpressed APO-1 and CD54 at very high levels. Immunohistologically, Apo-1 was detectable at low levels in a subpopulation of follicle center B blasts and, at higher levels, in sinusoidal B cells. APO-1 was undetectable in follicular mantle B cells and plasma cells. In isolated tonsillar B cells, APO-1 was expressed in CD10+ follicle center cells. In acute B lymphoblastic leukemia, chronic B lymphocytic leukemia, and Burkitt's lymphomas, APO-1 and CD54 molecules were immunohistochemically undetectable. Coordinate expression of these antigens was found in mediastinal B-cell lymphomas. The mode of APO-1 and CD54 expression was correlated in follicle center cell lymphomas (P < .0019), but less stringently in hairy cell leukemia. No association was found in plasmacytomas. This was in line with the differential expression of these molecules found in reactive plasma cells. Expression of APO-1 in B cells of different stages of differentiation and, correspondingly, in certain B-cell neoplasias might suggest a role of this molecule in the induction of B-cell apoptosis. This function might be influenced by CD54 and CD54-mediated signals.