Wide spread of pollutants present in the environment, has a profound deleterious effect on the interacellular homeostasis and may affect the gene expression. Among the most common hazards found in today's environment are the toxic metals and metalloids. Several metals have been found to produce acute and chronic tissue injury in humans and animals. It has been suggested that, one class of such toxic metals is the heavy metals as cadmium (Cd), Zinc (Zn), Copper (Cu), and Mercury (Hg). Exposure to one or more heavy metals (e.g. Cd, Cu, Ag, Zn, and Hg) had been found to elevate metallothionein concentration. The latter, was regarded as an inducible protein, and plays a key role in the homeostasis of essential metals such as Zn and Cu, and in the detoxification of some heavy metals as Cd, Hg, and Ag. When the toxic metals bound to metallothionein, they rendered inactive, and can not exert their deleterious effects on the sites of cell survival. Thus, metallothionein can help efficiently as a biochemical parameter in the diagnostic investigations for the toxicity resulted from heavy metals exposure, particularly in the industrial and polluted areas.