The Bureau of Mines, U.S. Department of the Interior, is investigating a hydrometallurgical procedure to conserve cobalt, nickel, and copper by recovering the metals from the matte byproducts of smelting Missouri lead ores. For want of adequate processing technology the mattes are now either unmarketable or marginally so. The procedure involves leaching the manganese dioxide and sulfuric acid to obtain copper, nickel, cobalt, lead, and manganese sulfates, and elemental sulfur as expressed by the following reaction: ms + mNO2 + 2h2so4 -> mso4 + mnso4 + s deg + 2h20, where m = cu, ni, CO, and pb. The leach solutions, containing near theoretical amounts of sulfuric acid and manganese dioxide at 85 deg to 100 deg c, convert up to 98 pct of the cobalt, nickel, and copper in the mattes to sulfates in less than 2 hr. As they are formed, the cobalt, nickel and copper sulfates are dissolved leaving a residue of lead sulfate, lead, sulfur, and gangue. The soluble metals are recovered from the leach solutions by caustic and sulfide precipitation, and the residue returned to the smelter for lead recovery. The manganese can be reclaimed from the spent leach liquor as manganese oxide and recycled to leaching.