The primary immune response of mice to the Thy-1 antigens was elicited by intravenous injection of thymocytes from H-2 compatible donors. The magnitude of the ensuing response judged by the number of plaque forming cells producing antibodies lytic for the Thy-1 bearing cells was influenced by some non-H-2 antigens present on the immunizing thymocytes. Absence of non-H-2 incompatibility resulted in a poor response whereas the presence of such an incompatibility allowed a good response. Genetic analysis revealed that the non-H-2 incompatibility affecting the anti-Thy-1 response is controlled by 1-3 independently segregating genes presumably determing molecules acting as carrier determinants for otherwise poorly immunogenic Thy-1 antigens. The carrier effect appeared to be influenced by the gene dosage of the carrier molecules on the immunizing cells. The data presented are discussed in the context of the current concepts of the mechanisms controlling various immune responses.