The staphylococcal superantigen toxic shock syndrome toxin-1 (TSST-1) is a potent inducer of IL-1 beta and TNF-alpha synthesis in human monocytes. As superantigens are high affinity ligands for MHC class II molecules, the induction of monokines by TSST-1 provides a biologically relevant model of MHC class II-mediated transmembrane signaling. In this study, we show that TSST-1 induces cytoplasmic protein tyrosine phosphorylation in the human monocytic cell line THP-1. This induction was greatly enhanced by cross-linking TSST-1 with biotin-avidin. The functional relevance of tyrosine phosphorylation induced by TSST-1 was demonstrated by the finding that three specific inhibitors of protein tyrosine kinases strongly inhibited the induction of IL-1 beta mRNA by TSST-1. These data suggest that protein tyrosine kinase activation plays a critical role in MHC class II-mediated transmembrane signalling by staphylococcal superantigens.