A strain of Streptomyces sp. isolated from soil was found to produce extra-cellular $eta$-lactamase associated partially to the cell growth. The $eta$-lactamase was purified from the culture supernatant through anmonium sulfate fractionation, ion-exchange chromatographies and gel filtration. The final purification fold and recovery yield were 57 and 6.2%, respectively. Molecular weight of the $eta$-lactamase was estimated to be about 67, 000 by SDS-polyacrylamide gel electrophoresis. The optimal reaction condition was at pH 7-8 and at 35-$45^{circ}C$. The $K_m$ and $V_{max}$ values of the enzyme for penicillin G were estimated to he 3 mM and $3 imes 10^3$ $mu extrm{M}$/min/mg protein, respectively. The purified $eta$-lactamase was classified to the class A enzyme hydrolyzing only penicillin.