This project concerns an interdisciplinary attack on several fundamental problems of cellular immunology. Our major objectives are: 1) To develop new methods for specific fractionation of lymphoid cells according to the specificity of their surface receptors using the method of fiber fractionation. This approach is being used to provide a quantitative description of clonal selection, to study morphological changes in lymphocytes, and to study cell-cell interactions in tissue culture. 2) To map the surface of lymphocytes in terms of their Ig receptors, lectin receptors, histocompatibility antigens and other distinctive antigenic markers. 3) To analyze the biochemical mechanisms by which lymphocytes are triggered to mature and divide by mitogens and specific antigens, including a description of the population dynamics, receptor-cytoplasmic interactions and surface nuclear interactions. 4) To isolate and characterize the major histocompatibility antigens in terms of amino acid sequence and to relate these sequences to the genetics, evolution and function of these antigens. 5) To determine the amino acid sequences and three-dimensional structures of a variety of lectins, lymphocyte products, and immunoglobulins by microchemical methods and by X-ray crystallography and to study the interactions of lectins with lymphocytes by electron microscopy.