When mice are sequentially immunized with two antigens to give an oligoclonal "double-binding" antibody response, there is a concomitant increase of "double-binding" cell surface receptors on their splenic lymphocytes. Competition studies suggest that the capacity to bind the two ligands, bovine pancreatic ribonuclease (EC 126.96.36.199) and a 2,4-dinitrophenyl (DNP) derivative, is a function of the same molecules. In ribo-nuclease-primed mice, an early response to bovine gamma globulin containing an average of 60 Dnp groups per molecule is the appearance of an increasing number of cells bearing surface receptors binding both ribonuclease and Dnp. Later, these double-binding cells are diluted by cells that bind Dnp, but not ribonuclease. The analogous phenomenon is observed when the two antigens are used in reverse order. While other reports suggest that there may be several different receptors in relatively undifferentiated cells from unimmunized mice, it seems likely that cells committed to antibody production carry a predominant multispecific cell surface immunoglobulin receptor.