The purpose of this study was to demonstrate those membrane receptor sites on mononuclear phagocytes of human CSF [cerebrospinal fluid] which provide additional evidence for their monocytic origin and function. Using a heterologous system, sheep red blood cells were coated with IgG[immunoglobulin G]- and IgM-fraction of the anti-Forssman-antiserum of rabbits. In another series of experiments, sheep red blood cells were additionally sensitized with fresh human serum as a source of complement. The possible inhibitory effect of human IgG on the uptake of red cell antibody complexes was tested. Washed and pretreated sheep red cells were added to different fresh CSF specimens from patients whose CSF exhibited no conspicious biochemical, serologic or cytologic alterations. The percentage of phagocytizing mononuclear phagocytes was evaluated. When the particular IgG EA [erythrocyte, antibody] reagent described was utilized, most of the mononuclear phagocytes consistently exhibited the IgG- and complement-receptor activity which selectively characterizes blood monocytes and related cells.