Dimeric hybrid protein molecule consisting of disulfide-linked beta (heavy) subunit of haptoglobin (Hp) and L (light) subunit of immunoglobulin G, was synthesized. Molecular mass of the hybrid was 62.0 kd, sedimentation constant B20 = 2.43. Hemoglobin binding activity of the beta X L hybrid as compared with that of native Hp accounted for 25%. In immunological cross-reactivity the hybrid was found to be deficient in some antigenic determinants present in Hp and in Hp beta chain. Hybridization carried out in three-component mixtures (Hp beta subunits, Hp alpha (light) subunits, IgG-L) showed, that besides reconstituted tetramers of Hp (2 alpha X 2 beta) always the hybrid beta X L was formed, even at an excess of Hp alpha, an excess of IgG-L prevented the formation of tetrameric molecules. Conformational similarity of Hp alpha and IgG-L as disulfide linked with Hp beta is reflected in the interaction of respective conjugates with hemoglobin.