Multifunctional systems must maintain homeostasis. Man is an ideal example of a system that constantly aspires to attain optimal regulation, even under the stress of severe disease. We assume that there are universal, interchangeable (as required) biologically active substances that regulate the system and try to keep it in balance. We propose that one of these substances is vitamin B12. Why vitamin B12? The list of organs and body systems in which vitamin B12 plays a functional role is constantly being added to. Vitamin B12 affects the normal growth of children, the peripheral and central nervous systems, bone marrow, skin, mucous membranes, bones, and vessels. It is possible that even when the serum cobalamin level is normal, treatment with vitamin B12 could correct defects caused by other biologically active substances. We call this phenomenon the "Master Key" effect. We suggest that this "Master Key" concept can be tested by treating diseases, such as recurrent stomatitis, various forms of hyperpigmentation, trophic ulcers, and burns, with vitamin B12, even if the B12 serum level is normal.