The two functional domains of a cloned human fibroblast NADPH-cytochrome P450 reductase have been expressed in Escherichia coli and purified on the milligram scale for crystallization studies. One domain contains the cofactor FMN-binding site and the other contains the binding sites for cofactor FAD and substrate NADPH. Crystals of both domains have been obtained by the microbatch method. The crystals of the FMN domain belong to the monoclinic space group P2(1) with unit cell dimensions of a = 39.3 Angstrom, b = 51.5 Angstrom, c = 47.8 Angstrom, and beta = 105.7 degrees and have one molecule in the asymmetric unit. Diffraction data up to 2.3 Angstrom were collected with a merging residual on intensity of 9.3%. The crystals of the FAD/NADPH domain belong to the orthorhombic space group P2(1)2(1)2(1) with unit cell dimensions of a = 55.9 Angstrom, b = 58.6 Angstrom, c = 131.1 Angstrom and have one molecule in the asymmetric unit. Diffraction data up to 2.6 Angstrom were collected with a merging residual on intensity of 8.0%. (C) 1996 Academic Press, Inc.