SummaryWe describe two cases of fatal septic shock causcd by Lancefield group G streptococci. Both were community-acquired and occurred in previously healthy adults. Both patients died in severe multiple organ failure despite prompt antibiotic therapy. Serological typing of bacterial cultures identified the T-protein antigen as serotype 300, but this was thought to be of little relevance to the pathogenesis; T-protein antigens are useful as epidemiological markers.M-typing and PCR (polymerase chain reaction) detection of the streptococcal pyrogenic exotoxin genes were found to be negative suggesting that this highly pathogenic organism may represent a new M-type.