Protein-protein interactions play a significant role in regulation of functional activity of cytochrome P450s. The aim of the present study was to elucidate the molecular interactions between steroidogenic enzymes CYP17 and CYP21 localized in endoplasmic reticulum membranes of adrenal cortex and involved in biosynthesis of corticosteroid hormones. In the present work, we demonstrate for the first time the direct interaction at molecular level between highly purified human recombinant cytochrome P450s in a mixed reconstituted system. The dependence of the interaction between CYP17 and CYP21 on concentration of the redox-partner - NADPH-cytochrome P450 reductase - is demonstrated, and it is shown that electrostatic interactions play a crucial role in the interaction between CYP17 and CYP21.