Fish, becauseof their position in the food chain, bioaccumulate in the body large amounts of pollutants from the environment, because of that they are regarded as the most suitable biological indicators of aquatic ecosystems. Melano-macrophage centers are clusters of pigmented cells that can be found as part of the stroma of hematopoietic tissue in the spleen, kidney and liver of fish, reptiles and amphibians. The appearanceof melano-macrophage centers varies according to species, and within species with respect to age, size, sex and physiological state ofan individual, and the ambient temperature. Melano-macrophage centers include three basic types of pigments: melanin, lipofuscin and hemosiderin. Origin and biochemicalrole of these pigments are not yet known. Melano-macrophage centers are considered phylogenetic precursors of embrionic centers of lymph nodes. It is assumed that the primary role of melano-macrophage centers is destruction, detoxification and recycling of endogenous and exogenic materials. Melano-macrophage centers can be taken as indicators of the health of wild fish populations, because the centers are increasing in number and size in inflammation, tissue damage and responses to infection. Melano-macrophage centers could be used as sensitive indicators of stress conditions in aquatic ecosystems, due to their participation in the various processes of disease, starvation and exposure to chemicals. It is necessary to pay attention to the variable parameters such as age, sex and size of specimens.