The effect of heavy metal concentration to the anaerobic sludge digestion process is studied by means of simple batch bioassay tests. Daily gas production rate and percentage volatile solid reduction decreased with increasing solids concentration. Analysis of the results by least square showed that gas production rate was directly proportional to the concentration of metals in sludge expressed as milliequivalent per litre. Threshold values for inhibition and toxicity limits, corresponding to 20% and 50% decrease in gas production respectively, are derived from the experimental results and can be used as a tool for diagnosis of potential toxicity.