Admixture of separated L and H polypeptide chains of 7S gamma-globulins under appropriate conditions has been found to result in the reconstitution of 7S molecules. The chains were mixed in 0.5 N propionic acid and when dialyzed into neutral aqueous buffers interacted to form reconstituted material in greater than 30 per cent yield. This material had sedimentation coefficients of 6S to 7S, a weight average molecular weight of 160,000, and its antigenic structure and electrophoretic properties were the same as those of 7S gamma-globulin. By the use of I(131) and I(125) labels on the different types of chains, combined with ultracentrifugation of chain mixtures in sucrose density gradients, the 7S product was found to contain both isotopes in ratios consistent with the presence of two L and two H chains. After hydrolysis with papain, the reconstituted material yielded products resembling S and F fragments. All of the isotope corresponding to L chains was found in the counterpart of the S fragment; the isotope corresponding to the H chain fraction was present in both fragments. The activity reconstituted from chains of a purified guinea pig antibody to f1 phage was found to be associated mainly with the 7S material. Hybrid molecules containing rabbit L chains and human H chains and of human L chains and rabbit H chains were formed by the same techniques of reconstitution. It was found that the interchain disulfide bonds of native 7S gamma-globulins could be broken and reoxidized, as could those of reconstituted 7S material. Reduced L and H chains mixed in propionic acid, dialyzed against neutral buffers containing mercaptan, then against neutral buffers in the absence of mercaptan, formed stable 7S molecules of molecular weight 160,000, which were dissociable only after reduction.