text: The motivation for this project is to learn how antigen receptors on lymphocytes function in regulating cellular differentiation. We will examine the behavior of antigen reactive cells under various conditions in vitro and in adoptive transfer systems. Receptor-antigen interactions on cell surfaces will be studied by fluorescence and electron microscopy. We will compare the biological and physical properties of specific antigen binding cells isolated before immunization at various stages in an immune response. At the same time, we intend to continue affinity labeling studies to compare common and variable structural features of the active sites of homogeneous anti-polysaccharide antibodies. BIBLIOGRAPHIC REFERENCES: Nemanic, M.K., D.P. Carter, D.R. Pitelka, and L. Wofsy, Hapten sandwich labeling II. Immunospecific attachment of cell surface markers suitable for scanning electron microscopy, Journal of Cell Biology 64, 311 (1975). Press, Joan L. Norman R. Klinman, Claudia Henry, Leon Wofsy, Terry L. Delovitch, and Hugh O. McDevitt, Ia antigens on B cells: relationship to B cell precursor function and to surface immunoglobulin, in Membrane Receptors of Lymphocytes, Eds. M. Seligmann, J.L. Preud'homme, and F.M. Kourilsky, 247-258 (Elsevier, New York, 1975).