Using rapid flow microfluorometry for analysis and sorting of cells, aspects of the following projects have been supported during the previous year: (1) characterization of cell surface antigens on T cells from normal fetal and adult murine thymuses and from thymuses of adult radiation bone marrow chimeras; (2) characterization of patterns of lectin binding to murine thymocytes; (3) characterization of new procedures for cell purification; and (4) characterization of a dual laser technique for simultaneously analyzing two independent cell surface markers on individual cells.