The term compound receptors (C.R.) is used here to describe reversible molecular complexes in the cell membrane which attain their final biologically active structure by rearrangement and assembly of several structural subunits. The C.R. to be discussed involve the participation of the glycoproteins belonging to the major histocompatibility complex (MHC), notably of class I molecules which are themselves reversible compounds of a heavy chain and a light chain (beta 2-m). The main thesis of this discussion is the postulate that the fundamental immunological phenomenon known as MHC restriction is due to the formation in the membrane of reversible C.R. with additional roles in the physiology of the cell. The interaction between MHC class I molecules and insulin receptor molecules will be mentioned as an illustration of the general hypothesis.