A haemolysin produced by Actinomyces pyogenes ATCC 8164 was purified from culture supernatant by ammonium sulphate and polyethylene glycol precipitation, ion-exchange chromatography on DEAE-Sephacel, and fast-protein-liquid-chromatography on Superose 12 prep grade. The purified haemolysin, designated as pyolysin, displayed a single band on poly-acrylamide gel electrophoresis, indicating a molecular weight of 55000. Additionally, using gel filtration, the same molecular weight was estimated. Further studies of the eluate of ion-exchange chromatography using isoelectric focusing also revealed a single protein band at pH 9.38 with haemolytic activity. A specific antiserum produced against pyolysin inhibited the haemolytic activity. The purity of the isolated protein was also determined by Western Blot analysis with antiserum obtained from a cow inoculated with culture supernatant from A. pyogenes and Peptococcus indolicus. The isolated pyolysin appeared to be heat-labile and displayed cytotoxic effects on poly-morphonuclear leucocytes and on pTK2 kidney cells.