Abstract: We determined the interaction sites of the cytochrome P450's protein-partners: 2B4 (d-2B4) and NADPH-cytochrome P450 of reductase (d-Fp). While in operation, these proteins are forming the complexes. We used 4-4'-dithio(bisphenyl)azide linker for non-specific covalent coupling of d-2B4 complexes with d-Fp in Emulgen-913 - monomerized system. Covalently-linked peptides in this complex were identified with ESI-MS/MS. Several sites of these proteins' binding with each other were revealed. Based on them, a model of intermolecular protein interactions was created. The model includes 5 cross-linker-stabilized contact sites of d-2B4 with d-Fp involving the following peptides of d-2B4 and d-Fp: 1) d-2B4423-433 и d-Fp 102-109; 2) d-2B4324-336 и d-Fp570-585; 3) d-2B4327-336 и d-Fp452-464; 4) d-2B4 192-197 и d-Fp456-464; 5) d-2B4 134-139 и d-Fp406-425.Herein, in the latter two cases, the peptides of d-Fp are located in their inter-domain slit and stabilize protein-protein complex via nanoprobe cross-linker; therefore, the formation of d-2B4/d-Fp complexes in these sites may involve aminoacid residues d-Fp456-464 and d-Fp406-425 surrounding inter-domain slit.