All 40 tested isolates of M-type 57 group A streptococci gave the same, highly characteristic inhibitory pattern (P-type 614) when tested by deferred antagonism using a set of nine indicator strains. One component of this inhibitory activity was attributed to the production of a bacteriocinlike substance, streptococcin A-M57 (SA-M57). Production of SA-M57 was enhanced by the presence of blood and also by growth at an alkaline pH. Partially purified SA-M57 was obtained from culture supernatants by a combination of ammonium sulphate fractionation and column chromatography. On Sephadex G-100 two different molecular weight forms (SA-M57 alpha, greater than 100 000; SA-M57 beta, 33 000) were demonstrated. Both SA-M57 forms were protease and heat sensitive and had identical inhibitory activity against a collection of indicator bacteria. The adsorption to and rate of kill of sensitive cells by SA-M57 was enhanced in the presence of human plasma. Partially purified SA-M57 preparations were devoid of M-type 57 protein and curing studies showed that loss of SA-M57 did not correlate with loss of M protein production.