The suppressive effect of the cells on the bone marrow of the B-lymphocytic series on the production of antibody-forming cells to sheep red blood cells, observed on their addition into the culture of the spleen cells after Mishell and Dutton, was mediated only by the live cells capable of proliferation, and was independent of the histocompatible differences between the bone marrow and the spleen cells and of the preliminary immunization of the bone marrow donors.