Twenty human malignant solid tumors of various histologic types were tested for the presence of Fc receptor using cryostat sections or single cell suspensions of fresh tissue. Sheep erythrocytes sensitized by various amounts of rabbit IgG antibodies served as indicator cells (EA). All tumors possessed Fc receptor, but to varying degrees; eight reacted more strongly than normal spleen without any relation to histologic type. The tumors which gave the strongest reactions in sections also formed the highest percent of EA rosettes in suspensions, thus indicating surface localization of receptors. The reactions with spleen sections localized to the B cell and monocytic areas; the latter also showed high avidity in reactions with uncomplexed IgG. Rabbit antisera to tumors, spleen, and peripheral lymphocytes (polyvalent ALS) in inhibited the reactions, while a T-cell-specific ALS did not. Absorptions of the antisera with lymphocytes or tissue sediments of spleen and tumors removed the inhibiting activity, tissue sediments of muscle and kidney only reduced the titers. Again, results with spleen sections paralleled those obtained with tumor sections. Apparently, the tumor Fc receptor is very similar to the Fc receptors present in normal lymphoreticular tissues.