Our data on cytochromes P450 2B4 and b5 were cross-linked by soluble carbodiimide in catalytic complex support the idea of overlapping binding sites for cytochrome b5 and NADPH:cytochrome P450 reductase on the cytochrome P450 2B4 surface. Mass spectrometry revealed four amino acid residues of cytochrome P450 2B4 (Asp134, Lys139, Glu424 and Glu439) involved in cross-links with cytochrome b5.