Alteration of IgG by oxygen-derived free radicals has been implicated in an in vivo process which renders IgG autoantigenic and leads to the production of rheumatoid factor (RF) and the perpetuation of inflammation, as in rheumatoid arthritis (RA). In this study the impact of UV irradiation on IgG was investigated as well as the ability of RF to bind to UV-altered gamma globulin. Inhibition studies of the binding of ¹²⁵I aggregated human gamma globulin (AHG) to RF-coated sepharose beads show that UV-irradiated IgG is able to bind RF to the same extent as AHG. Binding studies to ¹²⁵I-C1q proved that UV-irradiated IgG could bind the first complement component, but also that the complement system could be activated as illustrated by the C3a generation. These results support the hypothesis that free radical damage to gamma globulins plays a role in the chronicity of the inflammatory reaction in RA.