Abstract Because C-reactive protein (CRP) has been identified as a component of circulating immune complexes from patients with inflammatory diseases, we sought to evaluate a potentially clinically important interaction of this acutephase protein with immunoglobulin or experimentally-prepared immune complexes in vitro. Highly purified human CRP was incubated with a variety of immunoglobulin substrates, including monomeric immunoglobulin G1 (IgG1), a polyclonal IgG, heat-aggregated IgG, and human serum albumin/anti-serum albumin complexes. We were unable to detect a significant binding interaction of radioiodinated CRP with any of these materials, using either polyethylene glycol (PEG) precipitation or sucrose density gradient ultracentrifugation. In contrast, binding of radioiodinated human C1q to both aggregated immunoglobulin and immune complexes was readily detected by these techniques. Incubation of radiolabeled CRP with serum samples from 22 patients with active inflammatory diseases and high levels of circulating immune complexes disclosed no difference in the amount of PEG-precipitable CRP when compared with serum samples from healthy individuals. However, a radiolabeled commercial preparation of CRP did result in some PEG-precipitable radioactivity after incubation wtth aggregated IgG. These findings provide no support for a biologically important binding interaction of CRP with immunoglobulin or immune complexes, and they suggest that highly purified preparations of CRP should be used in functional studies of this acute-phase protein.