A clonal analysis has been made of the murine BALB/C response to lactose-containing immunogens with respect to the affinity restriction in IgM expression. Monoclonal IgM and IgG antibodies were prepared from antilactosyl hybridomas generated from mice immunized with p-aminophenyl-beta-lactoside (PAPL) coupled to BGG or with a vaccine of Streptococcus faecalis (Strain N). Association constants for the binding of monovalent derivatives of PAPL were measured by quenching fluorescence. These derivatives carried a probe (2,4-dinitrophenyl or 1-dimethylaminonaphthalene-5-sulfonyl) that served to quench the protein fluorescence when the ligand was complexed with the protein. The central finding was that with both immunogens a restriction in the affinity of IgM was demonstrated since the highest values exhibited by IgG antibody exceeded by at least a factor of 50 the highest comparable values of the association constants for IgM antibody. It is suggested that the hypothesis of germ-line restriction, previously proposed as the basis for the affinity restriction of IgM, may also be applicable to the T cell receptor since its distinctive properties parallel those of IgM antibody.