A screening protocol has been developed to provide a rapid and repeatable assessment of the effect of toxic organic chemicals on anaerobic treatment processes. The protocol also provides information on the rate limiting biological reactions and the concentrations at which changes in reaction rate take place. General features of the protocol include relative ease of use, relatively short response time - days versus months, low labor requirements, use of conventional laboratory techniques, reproducibility among laboratories, and it builds on previously developed protocols and procedures. The screening protocol consists of three steps: STEP I is a pre-screen to identify the range of toxic impact of a chemical; STEP II is a short-term batch test to identify the effect of slug doses of chemical and to show which anaerobic reaction is inhibited first; STEP III is a semi-continuous culture test to identify the concentration at which specific inhibitory effects take place.