Previous investigations have provided evidence to suggest that C-reactive protein (CRP), an acute-phase reactant, binds to human monocytes at a membrane site that is either identical to or physically associated with IgG Fc receptors. To characterize further the relationship between monocyte CRP binding sites and IgG Fc receptors, monocytes were allowed to attach to surfaces coated with IgG or CRP and binding-site redistribution was assessed. Binding was measured by using protein-coated sheep erythrocytes (E). When attached to control (gelatin or albumin) surfaces, greater than 60% and 43% of monocytes formed rosettes with E-IgG and E-CRP, respectively. Following adherence to surface immobilized CRP, the proportion of cells binding E-IgG was unchanged; however, fewer than 20% of monocytes bound E-CRP. When attached to IgG-coated surfaces, fewer than 20% of monocytes formed rosettes with either E-IgG or E-CRP. In order to determine whether the unidirectional modulation of CRP and IgG binding sites was the result of CRP binding directly to a subclass of IgG Fc receptors, fluid-phase IgG-blocking studies were performed. When monocyte monolayers were preincubated with either monomeric or heat-aggregated IgG, a dose-dependent reduction in E-IgG binding was observed. In contrast, all concentrations of fluid-phase IgG failed to inhibit monocyte binding of E-CRP. These data indicate that CRP binds to human monocytes at a site physically associated with but distinct from IgG Fc receptors.