We propose to study the role of the cell surface in lymphoid cell differentiation by examining cell surface antigens determined by the Tla region of the mouse. This region controls a series of thymus-leukemia specific antigens called TL and a seris of lymphocyte antigens called Qa. Our approach will be to determine (1) how these cell surface antigens are controlled and expressed during lymphoid cell ontogeny and differentiation (2) how the Tla region genes regulate the expression of these cell surface components (3) what other cell surface antigens are controlled by the Tla region, and (4) what the relationship is between these Tla region antigens and those governed by the Major Histocompatibility Complex of the mouse. Our techniques will include immunization procedures, hybridoma production, serological definition of cell surface antigens by cytotoxicity testing, binding assays, and immunofluorescence, fluorescence-activated cell sorter analysis, and genetic linkage and complementation studies.