An enzyme was purified to homogeneity on a polyacrylamide gel by conventional chromatographic techniques from an extract of Lactococcus lactis ssp. lactis. The molecular weight of the enzyme was estimated to be 140,000, and it was composed of 2 homo subunits. The optimal activity was observed at pH6.0-6.5 and 30-35℃. The enzyme was almost completely stable between pH6.0 and 7.0, and 0 and 40℃. The enzyme seemed to a serine proteinase since it was inhibited by DFP and PMSF. The Km, Vmax and activation enemy of the enzyme for whole casein were 0.044%,1.10μg tyrosine eq./min/mg and 26,300cal/mol, respectively. αs1-and β-casein were degraded to some extent by the enzyme.