Murine myeloma immunoglobulin (IgA, κ) J539, which shows enhanced tryptophanyl fluorescence on ligand binding, and S10, which shows reverse-sign changes in tryptophanyl fluorescence on ligand binding (RLIF, see below), have been reduced, alkylated, and dissociated into their light (L) and heavy (H) chains. Two hybrid recombinants, H10L539 and H539L10, have been prepared and the 7S material has been isolated by chromatography. The binding behavior of these recombinants was studied with a number of ligands. Both recombinants showed activity with β(1→6) linked galactose ligands comparable to the native immunoglobulins. The ligand-induced fluorescence changes of the recombinants paralleled those of the heavy chain donor. For the recombinant H10L539, two different galactose-ligands caused fluorescence changes in opposite directions. It was quantitatively shown that binding of these ligands, nevertheless, took place in the same combining region. The idiotype of each recombinant resembled that of the heavy chain donor.