The affinity between purified rheumatoid factors (RF) and native or heat aggregated human IgG has been studied in vitro by polarization fluorescence in the presence and in the absence of D-penicillamine. The value of the dissociation constant was the same using native and heat aggregated IgG suggesting that binding to the aggregated protein is not dependent on the exposure of a new determinant lacking in the native molecule. The results obtained in the presence of D-penicillamine suggest that the concentration of the drug necessary to get a pronounced effect on the apparent dissociation constant of the immunocomplex between IgG and RF is not reached in vivo, in clinical situations.