Ethylene as plant growth regulator attracts an attention of many biologists. However, exactly what this molecule does in the cells is not clear. Its biological effects were explained by the formation of ethylene-receptor (ethylene-proteins) complexes onto the living cells. But what is the nature of a bond of such sophistical ethylene-receptor complex? Perhaps, one of the most delightful characteristics of theory of biological receptors is that just when we things we know all about this phenomena, a forgotten fact come along that exposes our state of ignorance. It is very known that ethylene at normal (ordinary) temperature and pressure is a very inert chemical. It cannot enter neither addition nor abstraction reactions. This is a fundamental chemical law and a bad (unpleasant) fact for biologists. Thus, the big question will remain: What is the chemical logic for so small molecule as ethylene to conjugate with so large molecules as proteins or peptides and then try to pass through so small membraneous pore to activate mostly putative enzymes? Many biological effects caused by chemical agents including ethylene are similar to those caused by exposure of plants to physical factors. Next question is: Such physical factors as X-ray, electrical or magnetic fields also require the conjugation with receptors to cause biological effects? Our data suggest that biological effects of ethylene are caused by formation of its free radicals in the free radical addition reactions of endogenous free radicals to an unsaturated bond. Free radicals of ethylene are reactive agents (they do not require any receptors) and can oxide many biological molecules, first of all membraneous lipids, also they can bind to many substances and thus it forms so called ethylene-receptor complexes. These bonded chemically non-active structures are consequences (not causes) of the free radical activation and action of ethylene.

Key words: ethylene, free radicals, receptors