The effects of bifunctional cross-linking reagents on the purified hexameric cytochrome P-450LM2 in an aqueous medium and on the proteoliposomal cytochrome P-450LM2 have been compared. In both cases, cross-linking is shown to result in the appearance of a range of additional protein bands in SDS electrophoretograms. The number and the positions of these bands seem to be similar in the solubilized and in the proteoliposomal cytochromes. No additional bands appear when the purified cytochrome P-450 was pretreated with 0.2%. Emulgen 913, which decomposes cytochrome P-450LM2 hexamers. The results indicate that the membrane-bound cytochrome P-450 can exist in the oligomeric (presumably hexameric) form.