Quantitative and qualitative analyses of the refringence phenomen in immunocompetent cells of various organs of rabbits and CBA line mice under conditions of immunological activity, as well as in those of children with ataxia-teleangiectasia, showed that immunoblasts contained predominantly non-refringent nuclei, mature plasmatic cells--mainly highly refringent nuclei, and young plasmatic cells occupied a position in the middle of the two. These data, as well as the absence of the refringence phenomenon in the cells of lymphosarcoma in man corroborate a direct relationship between the phenomenon and the process of differentiation of cells.