Recently, others have reported that the toxic effects of copper could be eliminated by the addition of sulfide to the digester in stoichiometrically equivalent amounts. It was desired to assess the generality of their report, both with regard to high concentrations of copper and the use of sulfides for control of toxicity of other heavy metals. To this end, the study was performed to evaluate the use and importance of sulfide in the control of toxicity produced by the heavy metals copper, zinc, nickel, and iron in the anaerobic treatment of primary municipal sludge. (Author)