There are an almost bewildering array of assays one can use to detect Fc-receptors, but we have focused, for the most part, on relatively rapid and convenient methods--those that have become standard, e.g., EA rosetting procedures or uptake of soluble antigen-antibody complexes. For most types of Fc-receptor studies these methods are quite adequate. We have also tried to stress the newer methodology of detecting Fc receptors, i.e., the use of anti Fc-receptor antibodies, including monoclonal antibodies. The advent of monoclonal antibody technology, described in detail by Galfrè and Milstein, is changing the approach of many immunological procedures, and detection of Fc receptors is no exception. There are, it should be noted, many newer Fc-receptor methods, e.g., a solid-phase radioimmunoassay and a quantitative fluorometric assay, and methods such as these may be the most appropriate ones to choose in a particular experimental situation. In addition, we have not stressed "functional" Fc-receptor detecting assays, for example, antibody-dependent cell-mediated cytotoxicity (ADCC), which can be used to monitor the activity of Fc receptors in a whole cell population, rather than in single cells.