The aim of this study is to elucidate characteristics of ${eta}-lactamase$ inhibitor produced by Streptomyces sp. SMF-19 isolated from soil was found to produce proteinaceous extracellular ${eta}-lactamase$ inhibitor. The ${eta}-lactamase$ inhibitor was purified through ammonium sulfate fractionation, gel filtration, anion exchange chromatography and fast performace liquid chromatography. The molecular weight of the ${eta}-lactamase$ inhibitor was estimated to be about 48,000 by SDS-PAGE. The mode of inhibition against penicillin G as a substrate was uncompetitive. The ${eta}-lactamase$ inhibitor was stable in wide pH range but unstable at high temperature above $50^{circ}C$.