The disodium salt of quinacillin (3-carboxy-2-quinoxalinylpenicillin) is highly effective in protecting mice experimentally infected with penicillin-sensitive or penicillinase-producing strains of Staph. aureus. Like other penicillinase-resistant penicillins, it is very much less effective against infections initiated with methicillin-resistant strains obtained from clinical sources. Quinacillin is not toxic to laboratory animals except when large doses are repeatedly administered to rabbits; under these conditions toxic manifestations are attributable to changes in gut flora. Quinacillin is poorly absorbed from the gastrointestinal tract in man and animals, but is well absorbed and distributed when administered by injection. It is rapidly excreted in urine, and probably bile, apparently unchanged. Quinacillin, like benzylpenicillin, is only moderately bound to serum proteins, and penetrates into inflammatory foci.