The results obtained by the use of sulfanilamide and of prontosil1in the treatment of infections produced by hemolytic streptococci,2and, to a lesser degree, of infections produced by type III pneumococci,3leave no doubt as to the therapeutic efficacy of these compounds; but at the same time an adequate explanation of the mechanism of their action is lacking. Colebrook, Buttle and O'Meara4found sulfanilamide to be bactericidal against the hemolytic streptococcus in vitro and, to a limited degree, in vivo. They also recognized a discrepancy between the remarkable therapeutic results obtained and the limited bactericidal activity observed and suggested that the enhanced bactericidal action of the blood was supplemented by that of the tissues of the whole animal. Long and Bliss,5on the other hand, considered the stimulation of phagocytic activity of the polymorphonuclear leukocytes and of the monocytes of paramount importance in the