A patient with streptococcal toxic shock-like syndrome (TSLS) caused by Streptococcus agalactiae (group B Streptococcus) is described. The patient had all of the defining criteria for toxic shock syndrome (TSS), including fever, hypotension, erythematous rash, desquamation, and involvement of multiple organs. Neither Staphylococcus aureus nor group A streptococci were recovered, but vaginal and urine cultures yielded group B streptococci. The group B streptococcal isolate gave negative results in antibody tests for the production of TSS toxin 1, staphylococcal enterotoxins, and streptococcal scarlet fever toxins. However, the strain produced a novel pyrogenic toxin. This toxin, purified from culture fluids by ethanol precipitation, isoelectric focusing, and reverse-phase high-pressure liquid chromatography, had a molecular weight of 12,000 and an isoelectric point of approximately 7.0. The purified toxin was pyrogenic in rabbits, enhanced the susceptibility of the animals to lethal endotoxin shock, and caused the proliferation of rabbit splenocytes; these properties define pyrogenic toxins. When given to three rabbits via a subcutaneous miniosmotic pump, the toxin caused TSS-like symptoms ending in death. Three additional group B streptococcal strains from patients with TSLS were tested and were found to produce a toxin with similar properties.