The present study compared the immune response to a test antigen, sheep erythrocytes, in hemocyanin treated versus control mice. Mice were assayed for each of four response parameters: plaque-forming cells, rosette-forming cells, serum 50% hemolysins, and serum hemagglutinins. Experimental mice received several daily 100 mg doses of Limulus polyphemus hemocyanin prior to test challenge with sheep erythrocytes. The mice subjected to this profound form of antigenic competition, produced plaque-forming cells in response to the sheep erythrocyte test antigen which were unexpectedly much more numerous than in control mice. Serum 50% hemolysin titers for hemocyanin-treated mice were consistently feeble, however, compared to those of corresponding control mice. Accordingly, the plaque-forming cells of experimental mice produced either less antibody per cell or an antibody of inferior hemolytic efficiency. Alternatively, a combination of these effects could have been operative in order to produce the paradox of very low 50% hemolysin levels in experimental mice with clearly elevated splenic plaque-forming cell responses.