To investigate the mechanisms underlying superantigen (SAg) stimulation, we analyzed the effect of SAg on monocyte responses with or without lipopolysaccharide (LPS). Addition of gamma interferon (IFN-) to unstimulated cultures induced a marked increase in the number of CD80⁺ monocytes, which was inhibited by LPS through the action of interleukin-10. However, CD80⁺ monocytes began to increase before IFN- production, observed after 9 h of stimulation with staphylococcal enterotoxin B (SEB). SEB selectively increased the number of apoptotic CD80 monocytes, whereas LPS-treated monocytes were resistant to the apoptotic action of SEB. This SEB-induced killing was abrogated by anti-CD95 monoclonal antibody (MAb) ZB4 and anti-CD95 ligand (CD95L) MAb NOK2, suggesting a CD95-based pathway of apoptosis. Furthermore, the numbers of SEB-induced CD80⁺ monocytes were partially decreased by anti-CD119 (IFN- receptor) MAb and by anti-CD95L (NOK2) MAb. The CD30 expression of CD27^high T cells induced by SEB was increased by agonistic anti-CD95 (CH11) MAb. Together, our findings showed that SEB-induced monocyte apoptosis is closely associated with the enrichment of CD80⁺ monocytes generated before IFN- production, followed by up-regulation of CD80 by IFN-, and that LPS has negative effects in both cases. These results also suggested that induction of monocyte apoptosis is an important mechanism by which SAg exerts its anti-inflammatory effects.