It was shown that blood leukocytes and peritoneal cells of mice specifically bind serotonin (5-hydroxytryptamine) in amounts which exceed manyfold its binding by the cells of thymus, lymphatic nodes, spleen, Peyer's patches and bone marrow. The nature of this binding suggests that peritoneal cells and blood leukocytes contain cell populations (apart from those already known ones) capable of binding and trapping of serotonin.