AbstractFive tests, based on methods currently in use for measuring the inhibition of rates of respiration, nitrification, and growth of bacteria, are evaluated for use as screening tests for assessing the treatability of settled sewage and toxicity of industrial effluent. A treatability factor is derived for each test for a series of settled sewages and a toxicity factor for industrial effluents by comparison with control standards. The Microtox test gives greater sensitivity, repeatability, and precision, but possible problems in the interpretation of the results warrant further comparison with the remaining tests, all of which show reasonable promise.