The gene encoding human beta-carotene-9',10'-oxygenase, which cleaves the 9',10' double bond in beta-carotene into beta-apo-10'-carotenal, was cloned and expressed in Escherichia coli. Under aqueous conditions, the optimum organic solvent for the formation of detergent micelles was toluene. The optimum pH, temperature, detergent type, and the optimum concentrations of detergent, substrate, and enzyme for beta-apo-10'-carotenal production were 8.0, 37 degrees C, Tween 40, 2.4%, 300 mg beta-carotene/l, and 0.25 U/ml, respectively. Under the optimum conditions, 43 mg beta-apo-10'-carotenal/l was produced after 21 h with a conversion of 14%. This is the first report to describe the enzymatic production of beta-apo-10'carotenal.