α-Fetoprotein (AFP), depending on the surrounding conditions, exerts different functions by different mechanisms. (1) A regulatory effect on the concentration of the unbound form of its various ligands (e.g. fatty acids, estrogens, phytosteroids). Thus, numerous studies have demonstrated that fatty acids, particularly the polyunsaturated fatty acids, modulate positively or negatively many steps of the action of various steroids and many enzymes involved in the transduction of membrane-triggered signals. (2) Different conformations (holoforms) of AFP, depending on the nature and concentration of the ligand(s) bound, might influence the binding of the protein to specific receptor(s) and as a consequence its biological properties (internalization, action on the membrane signal transduction pathway). (3) Regardless of the mechanisms, proposed in points 1 and 2, the protein can exert effects associated with other signals such as growth factors. The nature of the associated growth factor(s) and AFP ligand(s) can orient cells towards multiplication or differentiation