Affinity capillary electrophoresis (ACE) has been used to investigate the epitope on the human immunodeficiency virus (HIV) core protein p24 recognized by the monoclonal antibody (mAb) 13-102-100. The affinity of a series of peptides with N- and C-terminal truncations of the epitope sequence determined by mass spectrometry was studied. The peak area change assay was used for the study of the interactions of the mAb with those peptides, exhibiting tight binding to the mAb, and the migration time shift assay was used to probe the relative affinities of peptides showing weak binding to the mAb. The experimental results show that the monoclonal antibody 13-102-100 recognizes the peptide VHPVHAGPIAP with highest affinity. Smaller peptides incorporating only part of the epitope, however, are recognized to some extent in the ACE experiments.