Monoclonal antibodies specific for the loop determinant (residues 64-80) of hen's egg white lysozyme demonstrated an immunoglobulin class restriction. Only IgM response against the loop could be evoked in mice, irrespective of whether the immunogen was the intact native lysozyme as such, or the loop peptide covalently conjugated to a synthetic carrier, poly-DL-alanyl-poly-L-lysine (A-L). Despite the fact that in polyclonal antisera from mice immunized with lysozyme, the ELISA-titre of anti-loop reactivity was very low, 26% of the total anti-lysozyme response could be accounted for by the loop when expressed as the percentage of anti-lysozyme hybridoma colonies producing monoclonal antibodies reactive with the loop. The results can be interpreted either as determinant controlled isotype restriction, or alternatively, as an affinity restriction leading to the phenomenon that antibodies of isotypes other than IgM are formed, but are of too low avidity to be detected by the ELISA method.