A laboratory method to monitor the persistence or degradation of toxicity in complex effluent/receiving water mixtures was developed and preliminarily field-validated. The research developed a method for possible future use in the National Pollutant Discharge Elimination System effluent permitting process that recognizes differential rates of toxicity degradation in effluent/receiving water mixtures. Initial studies were made with the toxicant 2,4-dichlorophenol (DCP) added to a natural receiving water in the laboratory. After incubation under conditions similar to the field, toxicity (as measured periodically by Microtox R) paralleled the degradation (or persistence) of the toxicant as measured chemically. Loss of DCP in the laboratory was similar to predicted field-loss rate. Receiving waters and industrial and municipal effluents were then incubated in the laboratory in proportions similar to these observed in the field. Following development of test protocol with the DCP and effluents, the laboratory unit was field-validated using chlorine in Mississippi River water in the field and in the laboratory. The simple laboratory method can be used to predict the persistence (degradation) of effluent toxicity in natural receiving waters. (Copyright (c) American Society for Testing and Materials 1988.)