Up-regulation of TLR4 in monocytes by S pyogenes supernatants is solely due to superantigens. Dilute (1/10) supernatant from a superantigen-producing strain of S pyogenes (H293) up-regulates TLR4 on human monocytes. Disruption of smez gene (strain H377) abrogates TLR4 up-regulation. Complementation with smez (strain H432) restores TLR4 up-regulation beyond that of wild-type (WT). ^*Denotes significant difference from H293 parent strain. Mean and SD of triplicate monocyte cultures; results are representative of separate experiments in 3 different individuals.