Purified staphylococcal α-toxin (molecular weight approximately 36 000) was mildly digested with trypsin, yielding two components by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. A fast-moving component (molecular weight 17 000 ± 5%) which is relatively resistant to tryptic digestion and a slow-moving component (molecular weight 20 000 ± 5%) which tends to aggregate. The fast-moving component was highly purified by means of combined procedures of column chromatography on Sephadex G-200 with zone electrophoresis on starch. The purified fast-moving component retained a high degree of lethal toxicity for mouse but lacked hemolytic and dermonecrotic activities, whereas the slow-moving component proved to be a nontoxic polypeptide. The lethal toxic fragment was antigenically active showing partial immunological identity with the parent α-toxin and stimulated the formation of antibodies capable of neutralizing the lethal action of α-toxin in vivo. Some physical properties and the amino acid composition of the purified lethal toxic fragment have been compared with those of native α-toxin.