Using rapid flow microfluorometry for analysis and sorting of cells, aspects of the following projects have been supported during the previous years: 1) further characterization of a subclass of B lymphocytes which have a critical role in humoral immune responses; 2) expression of Ia antigens on macrophages, B lymphocytes and thymocytes; 3) characteristics of liposome-cell interactions; 4) expression of cell surface antigens coded for by xenotropic murine luekemia virus. BIBLIOGRAPHIC REFERENCES: Weinstein, J. W., Yoshikami, S., Henkart, P., Blumenthal, R., and Hagins, W.A.: Liposome-cell interaction: Transfer and intracellular release of a trapped fluorescent marker. Science 195: 489-492, 1977. Ahmed, A., Scher, I., Sharrow, S. O., Smith, A. W., Paul, W. E., Sachs, D.H., and Sell, K. W.: B-lymphocyte heterogeneity: Development and characterization of an alloantiserum which distinguishes B-lymphocyte differentiation alloantigens. J. Exp. Med. 145: 101-109, 1977.