Biological transformation of Cr(VI) to Cr(III) by enzymatic reduction may provide a less costly and more environmentally friendly approach to remediation. In a previous report a Cr(VI) resistant actinomycete strain, Streptomyces sp. MC1, was able to reduce Cr(VI) present in a synthetic medium, soil extract and soil samples. This is the first time optimal conditions such as pH, temperature, growth phase and electron donor have been elucidated in vitro for Cr(VI) reduction by a streptomycete. Chromate reductase of Streptomyces sp. MC1 is a constitutive enzyme which was mainly associated with biomass and required NAD(P)H as an electron donor. It was active over a broad temperature (19-39 degrees C) and pH (5-8) range, and optimum conditions were 30 degrees C and pH 7. The enzyme was present in supernatant, pellet and cell free extract. Bioremediation with the enzyme was observed in non-compatible cell reproduction systems, conditions frequently found in contaminated environments.