The U.S. Department of the Interior, Bureau of Mines, is conducting laboratory-scale research to determine the leaching conditions required with sulfuric acid for extracting base metal from a matte with a copper-to-nickel ratio of 6 to 1. The matte (61.2 pct cu, 9.7 pct fe, 21 pct s, and about 5 oz of precious metals per ton) was prepared from Duluth Complex (Minnesota) ore by smelting a bulk sulfide flotation concentrate. The matte was preleached at atmospheric pressure to selectively extract 20 pct of the nickel, and the undissolved solids were referred to as the matte residue. Autoclave leaching of the matte residue extracted over 95 pct of the nickel, copper, and iron. The remaining leach residue in the autoclave contained over 100 oz of precious metals per ton. The leaching conditions required were matte grind of greater than 98 pct minus 325 mesh, pulp concentration of 5 g/l, total pressure of 100 psig with oxygen at 150 deg c or 400 psig with air at 180 deg c, 50 g/l h2so4, 700 rpm agitator speed, and 6 hours of leaching time. The leaching time was decreased to 2 hours by increasing the agitator speed to 1,300 rpm. Sulfuric acid concentration greater than 50 g/l did not appreciably enhance the base metal extraction, but with less acid, residues with higher iron concentrations were obtained. Supersaturation of the liquor was prevented at 150 deg c by using a lixiviant that contained 30 g/l ni and 20 g/l cu.