Accumulation of erythropoietic, granulopoietic cell elements, a change in the ration and absolute count of the T, B cells, and "null" lymphocytes, as well as a sharp inhibition of primary immune response were found in the spleen of CBA mice treated with antierythrocyte sera. The inductive phase of immunogenesis proved to be sensitive to the action of antierythrocyte sera. The data obtained apparently demonstrated the existence of a functional relationship between the immunocompetent spleen cells and their non-lymphoid hemopoietic microenvironment.