The recently discovered tetraspanin superfamily comprises a group of cell-surface proteins that are suggested to be involved in cell activation and signal transduction as well as in cell adhesion, motility, and metastasis. In this study, we have assessed the expression of two tetraspanins, CD53 and CD63, and two principal leukocyte adhesion molecules, CD11b and CD62L, on human apoptotic neutrophils. After aging of human neutrophils for 20 and 40 h in vitro, apoptosis was analyzed by light microscopy and flow cytometry. The binding of monoclonal antibodies directed against CD11b, CD62L, CD53, and CD63 on apoptotic and nonapoptotic cells was determined by dual-color flow cytometry. Aging of neutrophils in vitro resulted in a significant (P < 0.05) down-regulation of expression of the selectin CD62L, and a significantly increased expression of the two tetraspanins CD53 and CD63. The selective analysis of apoptotic versus nonapoptotic cells proved that both the increased expression of the tetraspanins and the loss of CD62L were restricted to the apoptotic subpopulation. An identical pattern of surface molecule expression was detected at 12 h after induction of apoptosis by an agonistic anti-Fas IgM monoclonal antibody. Further studies are required to clarify whether tetraspanins participate in the recognition of apoptotic circulating or extravasated neutrophils by macrophages.