Derivatives of the violet, iron-containing acid phosphatase of pig allantoic fluid have been prepared in which one of the two iron atoms present in the native enzyme has been replaced by zinc, copper or mercury. The derivatives so formed are enzymatically active: the Zn-Fe, Cu-Fe and Hg-Fe enzymes have specific activities of about 80%, 25% and 17% respectively, of the maximum specific activity of the Fe-Fe enzyme in the standard assay at pH 4.9 with p-nitrophenyl phosphate as substrate. In contrast to the Fe-Fe enzyme, the mixed metal derivatives are not rapidly inactivated by H2O2. Visible absorption spectra of the derivatives confirm that all of the visible absorption of the Fe-Fe enzyme is due to one of the iron atoms. Attempts to prepare an active Cu-Cu enzyme were unsuccessful. © 1984.