Protein precipitated from the sera of 15 patients with rheumatoid arthritis (RA) by increasing concentrations of polyethylene glycol 6000 (PEG) was compared with that from 20 normal controls. The wide range of precipitable protein obtained was only significantly different for the two groups when a 4% PEG concentration was used. To increase the discrimination of the procedure, the precipitated protein was measured for IgG, IgM, C3 and C4 using immunospecific antisera and laser nephelometry. The data obtained from each of these procedures did not increase the specificity of the technique as a means of distinguishing between patients and controls. As part of an investigation of the effect of penicillamine on immune complexes, penicillamine was incubated at 37 degrees C for 24 and 48 h with sera containing immune complexes from RA patients. When the immune complexes were precipitated with PEG, it was found that incubation, by itself, reduced the amount of PEG precipitable protein. The results of in vitro incubation with penicillamine were very variable.