Metallothioneins (MT) are the widespread proteins in animal world. These proteins are characterized by the low invariability of their structure. The number of aminoacids is fixed in every animal group, that is 60 (or 61) aminoacids, 20 of which are the cysteins radicals what makes over 30% of the aminoacid composition. Such a big ammount of cysteins which include the reactive sulfhydryl groups -SH determinates the metallothionein’s functions. Metallothionein take part in the homeostasis of the ions of the metals which are necessary for the proper metabolism of the organism (zinc, copper), and they also take part in the detoxication of the tissue from toxic metals. Apart from these they also protect the tissue from the reactive oxygen species. Metallothioneins are present in high concentration in fast-dividing and transformating cells. Experiments indicate that MT involvement in the process of cell proliferation and differentiation. The presence in MT molecules of a large number of thiolic groups with nucleophilic properties renders them capable of binding not only metallic cathions but also reactive oxygen species and organic radicals (loose electron couples beside a suphur atom easily bind with elements having an electron hole). Numerous studies point out an MT influence on the process of apoptosis.