IgG-IgG and IgG-IgM complexes were isolated from one rheumatoid arthritis (RA) serum by affinity chromatography to immobilized F(ag')2 gamma of specific antibodies against unique determinants in the complexes. Both IgG nd IgM, when isolated from these complexes, contained the unique determinants. The circular dichroism (CD) spectrum of IgG differed from that of normal IgG at neutral pH. At pH 3 both IgG and IgM displayed normal CD spectra, and only one third of the molecules now had affinity for the immobilized ligand. The molecules with affinity at pH 3 exhibited an abnormal CD spectrum at pH 3, and a normal CD spectrum was obtained only of those components that lacked affinity. One-third of the Fab gamma isolated from the IgG and IgG complexes with the unique determinants contained the unique determinants that were lacking in the rest of the same principal the Fc gamma fragments. The CD of the two Fab gamma preparations showed the same principal differences as the CD of the molecules with and without affinity to the ligand at acidic pH.