A great variety of cells express receptors for the Fc portion of immunoglobulins [1-41, Such Fc receptors (FcR) are present essentially, but not exclusively [3], on cells of the immune system, enabling interactions between immunocompetent cells and antibodies. In fact, evidence has accumulated that all cell types that compose the immune system express FcR [5-71 and that FcR exist for all immunoglobulin classes (including IgD)[8-121 and sub-classes [13-171. As it became evident that the expression and the specificity of FcR for various isotypes on different cells was complex and differed for one cell type to another, model cells were needed to address such questions as the multiple expression of FcR on a single cell, and the structural relationships between FcR on different cells. Tumor cell lines that express FcR provide an unlimited source of homogeneous cells for such studies [18-201. Most properties of FcR have therefore been inferred from studies of neoplastic cells. They will be reviewed herein.