In the last hundred years there was a large increase in enviromental pollution. Fish come in contact with pollutants, of which a large part bioaccumulate in their body and are therefore believed to be suitable as biological indicators of pollution. Melano-macrophage centers are clusters of pigmented cells that can normally be found as a part of hematopoetic tissue in spleen and kidney but also in the fish liver. The appearance of melano-macrophage centers can vary between species, organs and physiological conditions within the same species. There we can find three basic types of pigments: melanin, lipofuscin and haemosiderin, but the origin and biochemical role of those pigments varies and is not yet fully known. It is assumed that the primary function of those centers is destruction, detoxification and recycling of endogenous and exogenous cellular material. Apart from being "garbage collectors," melano-macrophage centers are considered as phylogenic precursors of germ centers of lymphatic nodes. It is experimentally shown that melano-macrophage centers, found in liver, are biologically suitable to be used as indicators of stress, but it is necessary to take into consideration variable parameters such as age, sex, sexual maturity and environmental factors.