Abstract Sera of 12 patients with rheumatoid arthritis (RA) who had positive IgM-rheumatoid factor (RF) tests were separated by use of immunoabsorbent columns (goat anti-human C3 [αHC3] and rabbit anti-human C1q [αHC1q]) and polyethylene glycol (PEG) precipitation-protein A affinity chromatography to isolate their immune complexes (IC). The isolated fractions were assayed for 19S IgM RF and 7S IgG RF by enzyme-linked immunosorbent assay (ELISA). The sera were further analyzed by preparative isoelectric focusing (IEF). The αHC1q and αHC3 columns were sequentially eluted with barbital buffer, 0.02 mol/L EDTA, 0.5 mol/L NaCl, and 1 mol/L propionic acid. All 12 patients had IgM RF and IgG RF in the EDTA fractions from both immunoabsorbent columns, but only IgM RF in the NaCl fractions. The PEG precipitation-protein A preparations were eluted with 0.5 mol/L glycine HCl and 3.5 mol/L MgCl 2. All 12 patients had significant titers of 19S IgM RF (⩾1:192) and 7S IgG RF (⩾1:96) in the acid-eluted fraction. Analysis of the sera by preparative IEF revealed IgM RF with a polyclonal spectrotype pattern with pH of 3.0 to 10.0, but predominantly acidic proteins with isoelectric points of 4.0 to 5.5. IgG RF were found in the same restricted spectrotypic pattern. These studies demonstrated that IC can be detected in the sera of patients with RA by isolation with αHC1q and αHC3 immunoabsorbent columns and PEG precipitation-protein A affinity chromatography. These IC are complement-fixing and contain IgM RF and IgG RF in high titers as detected by ELISA. The highest titers of 19S IgM RF and IgG RF were demonstrated in the pH range 4.0 to 5.5 by preparative IEF.