Abstract: A microbiological assay for chemicals, which uses a cell and a reducing dye to quantitatively measure inhibition of electron transport in the cell membrane as a function of chemicals in the substance being tested, is disclosed. This assay and method is reliable, simple, fast, and inexpensive, requires a minimum amount of durable equipment, and avoids the need for the use of live animals as the indicator organisms. The assay is particularly useful for testing for toxicity in food products, environmental, medical and industrial processes, sewage treatment, effluent, agricultural wastes, and chemical dumps.