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Studies were carried out in order to characterize the specificity of IgG sub-classes and IgG fragments for rosette formation using red cells and human mononuclear cells. Rosette formation of red cells coated with anti-D was inhibited by free IgG(1) and IgG(3); less inhibition occurred with IgG(2) and IgG(4). Red cells specifically coated with IgG(1) and IgG(3) by chromic chloride were bound to monocytes. Rosette inhibition of anti-D-coated red cells occurred with free Fc fragment of IgG globulin, and only partly with F(ab')(2). Inhibitory capacity of Fc fragments of IgG and gamma(1) heavy chain from heavy chain disease was reversed by the cleavage of disulfide bonds. No inhibition was noted with Fab, or with pepsin components II, III, or IV. These studies indicated that the mononuclear receptor was specific for IgG(1) and IgG(3). The peptide portion of IgG globulin which attached to the mononuclear cell appeared to reside in the N-terminal portion of the Fc fragment and also appeared to require the integrity of the inter-heavy chain disulfide bond. A specific receptor for C3 was not confirmed.