Varying titers of hemagglutinin antiserum were produced in mice of CBA strain both by active immunization with various dosage schedules of strain A spleen and by passive immunization with various doses of antiserum produced to strain A spleen in other CBA mice. Attempts to correlate the titer of the hemagglutinin antibody with the host's response to an A skin allograft revealed that the lower levels of actively produced antisera were correlated with sensitization to the graft, while, higher levels were correlated with prolonged survival of the graft. Conversely, the lower levels of passively induced antibody titers produced no discernible effect on the graft; however, higher levels again produced prolonged graft survival. Prolonged graft survival (enhancement) occurred when the hemagglutinin antibody reached titers in the range of 1:8192–1:16,384. It was noted that this is the range which begins to show excess antibody after maximum antigen-antibody combination has occurred. The possible implications of these findings are discussed.