Three hybridomas were selected which secreted monoclonal antibodies specific to a decapeptide determinant representing residues 103-112 of the tobacco mosaic virus protein ( TMVP ). A series of proteins from several strains of TMV which differ in the amino acid sequence in this region of the protein were used as probes for specificity analysis. The fine-specificity analysis was extended by assessing the binding of the antibodies with a panel of synthetic peptide analogues of the native decapeptide with amino acid substitutions at different locations. The binding of each synthetic peptide with each of the monoclonal antibodies was determined by the ability of the radiolabeled peptide to bind with the antibody. The binding of the decapeptide with antibodies was determined by equilibrium dialysis; the relative binding affinity of each peptide of the panel was determined by the capacity of the peptide to inhibit the binding between the antibody and the radiolabeled native decapeptide. The results demonstrated that a panel of synthetic peptide analogues constitutes a powerful tool for discerning the fine specificity of antibodies directed to a given determinant of a protein antigen. The data indicated that, although all of the antibodies recognized the same nominal decapeptide determinant, their binding with the different mutant proteins or with the synthetic peptides of the panel differed greatly, indicating dramatic differences in their fine specificity. The existence of such differences should be taken into consideration when assessing residues of a protein antigen that are involved in antibody binding. The differences which were found in monoclonal antibodies produced following immunization with the whole TMVP reflect differences which occur in heterogeneous serum antibody populations and point out the complexity of antigenic recognition even of as small an epitope as a decapeptide.