BALB/c mice were immunized with three A-myeloma proteins of BALB/c-2 or BALB/c origin (produced by plasmacytomas MOPC-315, MOPC-460, Adj. PC-22A). Noncross reacting antibodies were formed against Proteins-315 and 460, but the response to Protein-22A was marginal. Proteins-315 and 460 have anti-dinitrophenyl activity, and their reactions with the corresponding BALB/c antibodies were inhibited by dinitrophenyl ligands. It appears that antibodies can be formed in BALB/c mice against unique “idiotypic” determinants in the ligand-binding sites of some BALB/c mycloma proteins.