The reactions between purified anti-poly A. poly U and-poly I. poly C. antibodies (IgG and IgM), and synthetic and natural polynucleotides were visualized at the molecular level. This was achieved by the use of fine tungsten bidirectional shadowing of molecules adsorbed onto thin carbon films, combined with dark field electron microscopic observation. A progression was observed from monogamous multivalency (binding of a single multifunctional antigen molecule with several combining sites of the same antibody molecule simultaneously) (Crothers and Metzger, 1972, Immunochemistry, 9, 341-357), to aggregation. Different types of figures were observed, among which loops formed by the coiling of the antigen around a single IgM molecule were very frequently seen. The tendency of IgG antibodies to bind cooperatively to certain antigens was also noted. In contrast, cross-links were seldom encountered. The cross-reactivity of different polynucleotides was also assessed by a quantitative analysis. The length of antigen associated to an antibody molecule (either IgG or IgM) was also measured.