The relationship between the toxicity of 68 organic chemicals to fish and luminescent bacteria was evaluated. Chemicals for which 96-h median lethal concentration (LC₅₀) values had been measured for fathead minnows (Pimephales promelas) in flow-through tests at the U.S. Environmental Protection Agency (EPA) Environmental Research Laboratory-Duluth, Minn., were tested using the Microtox toxicity analyzer (Beckman Instruments, Inc.). The Microtox system measures the decrease in luminescence of the bacterium Photobacterium phosphoreum in response to a toxicant. The toxicity end point is the 5-min median effective concentration (EC₅₀), which is the concentration that causes a 50 percent reduction in light output.

The correlation between fish and bacteria toxicity for a composite of industrial chemicals and pesticides had an R² value of 0.65. However, the relationship of these two end points for a semihomologous series of industrial chemicals appeared to be firm, with R² being 0.96 for common alcohols. The feasibility of using the bacterial bioassay as a screening test in a cost-effective testing scheme is discussed.