The exocellular proteases of two Serratia marcescens strains (strains SF 178 and SH 186; both of serotype 06/014:H12 and bacteriocin type 18) were separated from the culture supernatants through precipitation with ammonium sulfate, followed by hydroxylapatite adsorption chromatography, Sephadex G-100 gel filtration, and DEAE-Sephadex A-25 ion exchange chromatography. The molecular weights amounted to 54,400 Daltons (SDS-PAGE electrophoresis). Both enzymes contained 1 g-atom of zinc and 7 g-atoms of calcium per mol. The amino acid sequences were essentially identical. Serologically, both enzymes cross-reacted strongly, suggesting similar antigenic determinants. The two enzymes were microheterogeneous; isoelectric focusing revealed two protein bands at pH 5.4 and 5.5, respectively. The optimal temperature for hydrolysis of azocasein was 30 degrees C. Both proteases revealed 2 optimal pH values (SF 178 = pH 6-7 and pH 8-10; SH 186 = pH 7 and pH 9).