Twenty clinical isolates of Staphylococcus aureus were examined to elucidate the virulence factors which are directly related to lethality in a mouse septic model. Heat or formalin treatment of the organism abolished the lethal activity of the live organism during challenge intravenously administered via the tail vein. Nevertheless, injection of ten times concentrated culture supernatant fluid (SUP) showed lethal activity in the mouse. However, there was no lethality when SUP was heated at 60 degrees C for 15 min. To examine variations of SUP lethality among strains, we collected 20 strains of S. aureus from four different hospitals. Then, we compared several factors for SUP lethality, which were the extracellular toxins and enzymes, such as toxic shock syndrome toxin 1, enterotoxin A, B, D, and hemolysins (alpha,beta,gamma), and also cytotoxic activity to human polymorphonuclear leukocytes and Vero cells. No difference was found among these factors except cytotoxic activity to Vero cells. Furthermore, we compared two strains in a mouse septic model according to the grade of bacteremia and lethal events. We found that mortality was higher with challenge by the strain whose SUP was lethal in comparison to the strain whose SUP was not lethal, even though the viable bacteria counts in the septic blood in both strains were not significantly different. This strongly supports the possibility that extracellular products, not the cell wall components, of S. aureus play the key role in the lethal event in this mouse septic model. In addition, among the extracellular products, those which have cytotoxic activity to Vero cells may contribute to the lethality in sepsis caused by S. aureus in this murine model.