Experiments were done to determine if a compost-based sulfate- reduction system could be used to treat nickel-contaminated mine waters. Sulfate-reduction systems were established in columns containing acid-washed mushroom compost. Simulated mine waters containing 2,000 mg sulfate 1-1 and 50 to 1,000 mg nickel 1-1 were adjusted to ph 4.5 and pumped through the columns at flow rates between 15 and 25 ml h-1. Initially, almost all of the influent nickel was removed in the columns by sorptive and ion exchange mechanisms. The nickel removal rate then dropped to 18 to 30 mg nickel day (7.8 to 12.8 Nmol g-1 compost day), where it remained relatively constant. The mechanisms responsible for the low and sustained rates of nickel removal on unamended compost are unclear. When sodium lactate was added to the inflow, sulfate reduction rates between 250 and 650 nmol day cm-3 compost were obtained and a sevenfold increase in the nickel removal rate was observed. The maximum nickel removal rate observed was 540 mg ni kg-1 compost day (92 nmol ni g-1 compost day) for columns receiving 1,000 mg ni l-1.