Mice were sensitized subcutaneously with both live staphylococci and Freund's incomplete adjuvant. They were challenged intravenously with the cocci one, two, and three weeks after the sensitization, and the number of cocci or abscesses in both kidneys was counted. After the intravenous challenge more cocci or abscesses were found in the kidneys of mice previously sensitized with the cocci than in those of untreated mice. Such increase in the number of cocci or abscesses in the kidneys of sensitized mice was prevented by subcutaneous injection with staphylococcal toxoid or killed vaccine prior to intravenous challenge with live cocci. These results suggested that the hypersensitive state to staphylococci might have been removed from the sensitized mice by the administration with the toxoid or vaccine.