The optical activity of soluble antigen-antibody complexes was studied. It was found that levorotation increases in the formation of skin reactive BSA-anti-BSA complexes, whereax it does not change on the formation of the Ag2Ab complex which is inactive in the skin reaction. The increase of levorotation was observed by the formation of skin reactive soluble complexes composed of synthetic precipitating R3'-dye and homologous antibody, whereas it does nto change on the formation of simple hapten (R)-anti-R complexes. These results support our hypothesis that the toxicity of soluble complexes is due to molecular changes brought about in the antibody molecules.