In this U.S. Bureau of Mines research, platinum-group metal (PGM) flotation concentrate from the Stillwater Complex, Montana, was subjected to biooxidation using thiobacillus ferrooxidans in an effort to break down the sulfide minerals and liberate the associated PGM's for subsequent chemical leaching. Bacterial treatment oxidized up to 94 pct of the sulfide present, destroyed the PGM-bearing pentlandite [(ni,fe)9s8] mineralization, and dissolved most of the nickel in the concentrate. Results showed that increased sulfide oxidation during the biological stage led to improved PGM recovery in the subsequent chemical leaching stage. Cyanidation at 80 deg c proved to be the best chemical leachant tested. It removed 76 pct of the palladium, 94 pct of the rhodium, and 97 pct of the gold, but only 34 pct of the platinum from the biooxidized concentrate.