Cell suspensions from the spleens of normal mice or mice injected with sheep erythrocytes were separated on a discontinous bovine serum albumin density gradient. Four bands or subpopulations were obtained and were assayed for antibody-forming cells, and for antigen-sensitive precursor cells. The antibody-forming cells were assayed by the hemolytic plaque assay and the antigen-sensitive precursors were assayed by the number of plaque-forming cells which developed after 3 or 5 day's culture with antigen. It was found that both antibody-forming cells and their precursors were present in the denser region of the gradient when spleen cell suspensions were taken from unimmunized mice. In contrast, both antibody-forming cells and precursors floated to the top in cell suspensions from mice sacrificed 1, 2, or 3 days after antigen injection. The change in density was detectable as early as 12 hr and was complete by 18 hr. The cell which changed in density was specific for the antigen that brought about that change. The significance of these findings is discussed.