Discharge to surface waters for disposal of concentrate from a membrane separation process requires point source discharge permits under at least Federal regulations (NPDES) and usually State and/or local regulations. These permits generally require that discharges not contain toxic substances in toxic amounts. Toxicity is currently determined by the bioassay testing process with whole effluent (undiluted concentrate) and with specifically prescribed standard test organisms. In Florida, toxicity is defined as 1/3 of the concentration lethal to 50% of the test organisms in 96 hours of exposure (1/3 LC50). Concentrate from a 6.0 MGD Reverse Osmosis Treatment Plant in Jupiter, Florida, was found to be toxic to the Possum Shrimp (Mysidopsis bahia) according to the 1/3 LC50 definition. The plant treats 8.0 MGD of brackish water drawn from the confined Floridan Aquifer, producing 6.0 MGD of potable water and 2.0 MGD of concentrate at approximately 20,000 PPM TDS. More than 30 bioassay tests and extensive chemical analyses were conducted with the concentrate in an effort to identify a specific toxic element or elements. This extensive testing and analysis did not reveal the presence of toxic substances in the concentrate. Literature review and specific bioassay testing indicated that the test organism, Mysidopsis bahia is very sensitive to ion concentrations and ion ratios. This research indicated that the mortality observed in bioassay tests was a result of the ionic concentrations and ratios of the concentrate being considerably different than those of natural seawater. The result of this research suggests that current bioassay testing procedures do not produce valid results for whole effluent brackish concentrates. Since these testing procedures are the only acceptable means of demonstrating the absence of toxicity to regulatory agencies, discharge of concentrate to surface waters will not be evaluated fairly. Consequently, this very cost-effective means of reusing concentrate is generally not available to water supply systems planning to use membrane separation technology.