A series of peptides, produced from peptide 68–88 by selective enzyme cleavage, were used to define the amino acid sequences reactive with Lewis rat anti-peptide 68–88 antibody. One hundred per cent of 38 rats immunized with peptide 68–88 recognized a B cell determinant residing in the region of residues 80–88. The three carboxyterminal amino acids, residues 86–88, appeared to confer conformation on this determinant. In roughly two–thirds of the rats, the reactivity with this major B cell determinant was equal to the reactivity with the immunizing peptide. In these rats the anti-peptide 68–88 antibodies were of the IgG2 class. A third of the rats recognized an additional determinant(s) and in these the predominant class of antibody was IgG1.