Abstract Acidic metal-contaminated drainages are a critical problem facing many areas of the world. Acid rock drainage results when metal sulfide minerals, particularly pyrite, are oxidized by exposure to oxygen and water. The deleterious effects of these drainages on receiving streams are well known. To address this problem, efforts are being made to use biological processes as an innovative, cost-effective means for treating acidic metal-contaminated drainage. Biological sulfate reduction (BSR) technology can be adapted to diverse site conditions and water chemistry. The Lilly mine near the community of Elliston, Montana, illustrates some of the specific conditions that can challenge effective application of BSR technology.