Hexavalent chromium (Cr6+) is a common and toxic pollutant in soils and wastewaters. Conversion of the mobile Cr6+ to less mobile and less toxic trivalent Cr3+ can be achieved with conventional chemical reduction and its subsequent precipitation. Alternatively, Cr6+ can also be reduced to Cr3+ by microorganisms and has potential for application in decontamination of industrial effluents. During our studies on chromium toxicity in actinomycetes, a strain of Streptomyces griseus was found to grow and reduce highly toxic Cr6+ to less toxic Cr6+ from media containing 5-60 ppm, of Cr6+. The concentration of Cr3+ increased with simultaneous decrease in Cr6+ during growth with complete conversion in 3-4 days. Reduction was faster when addition of chromium (especially high concentrations of chromium) was done after 24 h growth than at the time of inoculation. Conversion of Cr6+ to Cr3+ was complete within 24-48 h of chromium addition with significant uptake in the biomass. Reduction of chromium by resting cells and effect of various factors on chromium reduction are discussed. (C) 2002 Elsevier Science Ltd. All rights reserved.