The s-type pyocin was purified from the lysate of the mitomycin C-induced Pseudomonas aeruginosa 90-2-2205 cells by the other of ammonium sulfate precipitation, DEAE-Sephadex A-50 chromatography and Sephadex G-200 gel filtration. The purity was confirmed by the polyacry-lamide gel electrophoresis. The molecular weight of the purified pyocin was estimated 180, 000 by gel filtration. The pyocin was analyzed to be a complex of some polypeptides by the SDS-PAGE. The pyocin was stable by heat treatment and at pH 6-7.5 by adding 10-3% gelatin and 0.2M NaCl to the 10mM Tris-HCl buffer (pH7.5). Its killing action against the sensitive cells was assumably a single hit process.