Studies described in this paper indicate that two anti-H-2Kk monoclonal antibodies, namely 27R9 (H-2.25) and 30R3 (H-2.5) recognize different H-2Kk molecules on the surface of lymphocytes. Initial experiments in support of this conclusion were cocapping experiments which showed mutual exclusiveness between H-2Kk molecules which bind either of the two monoclonal antibodies 27R9 (H-2.25) or 30R3 (H-2.5) whereas conventional anti-H-2Kk (H-2.23) alloantiserum binds to both types of H-2 molecules. This result was confirmed by experiments using solubilized H-2 antigen preparations to inhibit antibody binding to spleen cells. Preabsorption of the preparation with one monoclonal antibody did not remove its inhibitory activity for the other monoclonal antibody, and only partially removed its inhibitory activity for the conventional anti-H-2Kk serum. These results suggest that at least two antigenically distinct H-2Kk molecules arc controlled by the H-2K region. Subsequent blocking studies have indicated that the two different molecules are associated, to some extent, in the cell membrane. Furthermore, in 125I-protein A radioimmunoassay.each monoclonal antibody was found to bind to only half of the estimated total number of H-2Kk molecules recognized by conventional anti-H-2Kk sera. Several interpretations for the existence of the two classes of H-2Kk molecules are discussed.