AbstractFcγ receptor‐bearing U937 cells, when incubated in serum‐free buffer, were found to release spontaneously a suppressor material for pokeweed mitogen‐drigen IgG synthesis which could be retained on Sepharose 4B‐IgG immunosorbents. Immunosorbents coupled with IgM or F(ab′)2fragments of IgG were unable to retain the inhibitory activity of U937‐derived material, suggesting a binding specificity for the Fcγ fragment of IgG. This suppressor material corresponds therefore by definition to an IgG‐binding factor (IgG‐BF). The mechanism forin vitrosuppression of the antibody response by U937‐derived IgG‐BF was investigated. It did not interfere with cell proliferation and displayed maximum effect when added at day 3 of the culture period. Tested for its effect on IgG, IgM and IgA synthesis, IgG‐BF suppressed antibody production following a pattern specific for IgG. Finally gel filtration of the suppressor material gave rise to two peaks of inhibitory activity with an apparent molecular mass comprised between 30 to 40 kDa and 60