Mice irradiated with lethal doses of total body X-irradiation are very susceptible to Staphylococcus aureus. A difference in virulence between a mouse-virulent strain and a mouse-non-virulent strain which lacks cell-wall aggressin could nevertheless be demonstrated. The virulence of the mouse-non-virulent strain could be increased by adding a cell-wall preparation with aggressin-activity (DOCR) from a mouse-virulent strain to the inoculum. When injected together with a dose of bacteria lower than the minimum pus-forming dose a lesion-enhancing effect of DOCR from a mouse-virulent strain was also demonstrated in man.