An immunomodulating effect of ceruloplasmin on major components of the immunocompetent system, the natural resistance system and the specific immune response, was found. Ceruloplasmin influences the level of expression of specific markers of T- and B-lymphocytes (determined in various modifications of rosette-forming test), phagocytic activity of neutrophils and monocytes, and the activity of ''respiratory burst'' enzymes. The modulation by ceruloplasmin depends predominantly on the initial level of the studied immunological parameters, i.e., on the extent of the immune inflammation in human patients. Thus, ceruloplasmin is not only an antioxidant and a copper-transporting protein, but it interacts with immunocytes, influencing their biological activities. The observed immunotropicity of ceruloplasmin and its ability to interact directly with immunocytes and to modulate immune function on the cellular level confirms the existence of a universal molecular language for information exchange between cells of different nature and origin of an organism.