The [4Fe-4S] cluster-containing enzyme dihydroxy-acid dehydratase (DHAD) is susceptible to inactivation by dioxygen and active oxygen species including superoxide with an inactivation rate constant of 10(6) m-1 sec-1. Based on this property, DHAD was used to quantify and investigate the biological oxidant stress activity of various redox-cycling chemicals. Exponentially growing cultures of Escherichia coli were used as a sensitive source of the DHAD enzyme. The effects on DHAD of compounds with and without established redox activity, under aerobic and anaerobic (control) conditions were measured. Paraquat, juglone, nitrofurantoin, the nitrofuran related compound NF-963 which is 6,7-dihydro-3-(5-nitro-2-furyl-5H-imidazo-[2,1-b] thiazolium chloride), plumbagin, benzoquinone, duroquinone, hydralazine, and naphthalene inhibited DHAD activity and the concentrations required for 50% inhibition ranged from 3.5 microM for paraquat to 950 microM for naphthalene. Eleven other agents tested (including 4,4-dipyridyl which is a non-redox-cycling compound similar to paraquat and an extract and two compounds of plant origin) did not inhibit DHAD. The DHAD technique described is a useful means of detecting and comparing the oxidant-stress toxicity and mechanism of action of chemicals by a biological means on a quantitative scale relatable to paraquat.