Summary: A mouse myeloma (MOPC-315) producing immunoglobulin A, which specifically binds 2,4-dinitrophenyl-hapten, has been shown to form hapten-specific plaques by a hemolytic plaque-in-gel assay. Sheep erythrocytes coupled with dinitrophenylated bovine serum albumin and rabbit antibody directed to the heavy chain as enhancing serum were used. Virtually all of the tumor cells formed plaques. DNP-lysine sharply inhibited plaque formation at a concentration slightly in excess of the reciprocal of the binding constant of the myeloma protein.