The Bureau of Mines conducted laboratory studies to determine the feasibility of chlorine-oxygen leaching for treating low-grade zinc sulfide flotation concentrates. Recovery of zinc sulfides by flotation from complex fine-grained sulfide ores is substantially higher in low-grade bulk sulfide concentrates than in the high-grade cleaned concentrates required for smelter feed. However, there is no commercial process in the United States for the treatment of low- grade bulk zinc sulfide concentrates. Leaching experiments using reactors of 500-ml and 1-gal capacity were conducted on a bulk concentrate containing 19.3 pct zn, 35.6 pct fe, and 39.4 pct s. The effects of pressure, temperature, and percent solids on extraction were investigated. The chlorine-oxygen leaching procedure showed promise for treating the concentrates. Zinc extractions of 95 pct or more were obtained by leaching at 50 pct solids in the temperature range of 100 deg to 120 deg c. Zinc extractions by leaching with chlorine followed by oxygen at reactor pressures of 50 to 100 psig were independent of pressure.