The Bureau of Mines conducted column-leaching tests on 25 domestic mn ores to determine the feasibility of extracting mn by in situ or heap leaching with dissolved sulfur dioxide (so2) solutions. The ores tested were obtained from deposits in Arizona, Arkansas, California, Colorado, Maine, Minnesota, Nevada, and South Dakota. Dissolved so2 was found to be effective for leaching mn; extractions greater than 90 pct were obtained with solution application rates of 1 ml/min from uncalcined, minus 2.5- Plus 1.3-Cm ore pieces. Leaching these ores with solutions containing only 5-wt-pct so2 and no added h2so4 resulted in mn being selectively extracted over fe except when fe was present as goethite or siderite. Ca solubilization was inhibited by controlling the solution application rate (1 ml/min). The rate of leaching was found to be principally dependent upon the rate at which the leaching solution can penetrate the ore, since the solubilization of mn upon contact with so2 was observed to be rapid. Despite low water permeabilities of the ores (as low as 10-2 to 10-4 darcy), greater than 90 pct of the mn was extracted from even the largest ore pieces tested (approximately 10 cm in diameter). It appears, therefore, that leaching mn opens up the pores, allowing penetration of the leach solution into the ore pieces.