Sedimentation equilibrium experiments with NADPH-cytochrome P-450 reductase showed that increasing 1-O-n-octyl-beta-D-glucopyranoside levels promoted disaggregation of the flavoprotein. The reductase was monomeric at a molar ratio of detergent to protein above 10(3). Addition of N3-carboxymethyllumiflavin to the flavoprotein in the presence of 1-O-n-octyl-beta-D-glucopyranoside results in photochemically induced dynamic nuclear polarization (CIDNP) signals in the aromatic region. The CIDNP spectrum of the holoprotein shows sharp resonances due to histidine residues. On removal of FMN from the protein, CIDNP signals originating from a tyrosine residue appeared, suggesting that the tyrosine residue is exposed to solvent after the depletion of FMN. However, this tyrosine residue appears to become inaccessible to the external dye after full incubation of FMN-depleted reductase with FMN. This suggests that the tyrosine residue could be located in the vicinity of the FMN-binding domain which constitutes the active center of the reductase.