A series of affinity bromoacetyl derivatives of the dinitrophenyl hapten has been shown to react with purified goat anti-dinitrophenyl antibody preparations at tyrosyl and lysyl residues. By means of the reversible amino group modifying reagent citraconic anhydride, it was possible to show that the lysyl residue modified by the affinity labeling ragents was not essential to antigen binding. Hence this reactive lysyl residue is probably situated outside of, but close to the hapten combining site. Additional experiments revealed the existence of two distinct types of goat IgG, a population which reacts with the affinity labeling reagents exclusively through tyrosyl residues, and one which reacts exlusively through lysyl residues. There does not appear to be a third population of antibodies which reacts both with tyrosyl and lysyl residues.