Extracellular serine proteinase has been isolated from the cultural medium of Bacillus intermedius 3-19 using CM-cellulose chromatography and affinity chromatography on bacitracin-Sepharose. The specificity of the proteinase with a wide range of natural and synthetic substrates has been investigated. The greatest activity was observed with tripeptides containing C-terminal Leu or Phe. The enzyme was completely inhibited by diisopropylfluorophosphate and partly inhibited by thiol-specific reagents. It is concluded that B. intermedius proteinase is a thiol-dependent serine proteinase pertaining to the subtilisin group. The amino acid composition of the enzyme has been determined. The enzyme contains one to three 1/2 cysteine residues, one of which is supposedly a Cys residue. The N-terminal amino acid sequences of the protein is AQTVPYGIPQIKAPA-.