Streptomyces naraensis was inoculated into 100 ml of culture broth, containing 50 μCi of 65Zn, diluted with ZnCl2 solution to make 10-4 M Zn2+ ion, at 27°C for 5 days with shaking. 65Zn-labeled neutral proteinase from Streptomyces naraensis was prepared by the method described previously.B) The preparation was homogeneous by disc electrophoresis and contained 1 g-atom of zinc per mole of enzyme in calculation by radioactivity.It was suggested that the protein-bound zinc of neutral proteinase was not essential for enzymatic activity. Thus, this zinc was an essential component for the higher order structure of the protein, and the removal of zinc treated with EDTA* inactivated the enzyme. The enzymatic activity was maintained in the presence of calcium ion.