The Fc receptor activity in various tissues has been studied using hemadsorption to cryostat sections of fresh frozen tissue or rosette formation with single cells in suspension. Indicator cells (EA) were sheep erythrocytes sensitized with various amounts of rabbit IgG antibodies (A). Malignant tissues and cells showed very similar behavior to lymphoid tissue and peripheral blood lymphocytes and monocytes, although adsorption of non-complexed A to monocytic areas of spleen was quantitatively stronger than to B lymphocytic areas and malignant tissues. Receptor activity measured as hemadsorption and the percentage of rosetting cells varied with the degree of sensitization of EA with lymphocytes, e.g., from 58% rosettes using EA sensitized with 1 agglutinating unit of A, to less than 1% with EA 1/16 unit. This indicates that Fc receptor is not a property of distinct subpopulations of the reactive cell types. Apparently, the presence and availability of this receptor fluctuate in or on each cell. Treatment of tissue sections or single cells with neuraminidase in various concentrations showed a similar enhancing effect on malignant cells, lymphocytes, and monocytes. Apparently, the Fc receptors possessed by the three cell types are similar in nature and properties.