The thymus of mice and chickens contains at least four discrete populations of lymphoid cells: Two distinct cortical populations of small lymphocytes (early and late population), a hydrocortisone resistant and presumably medullary population of small lymphocytes, and a population of medium sized lymphocytes (prolymphocytes) (see Table I and Figure 3). These four cell types were identified with preparative cell separation techniques (e.g. cell electrophoresis, BSA-density gradient centrifugation, and velocity sedimentation) in combination with size distribution analysis. The combination of these techniques provides two-dimensional distribution patterns ('fingerprints') with high power of resolution. At present the two cortical populations of small lymphocytes cannot be identified as distinct populations by any other method. The physical parameters also provide useful markers for the identification and comparison of cellular subpopulations in animals of different ages, different strains, and to a certain degree even of different species. It is believed that each of these subpopulations is in itself heterogeneous in respect to antigen specificity, and it is proposed to call lymphocytes with different antigen specificity but identical physical characteristics 'isotypic lymphocytes'. The medium and large thymocytes serve as progenitors of the small thymic lymphocytes, as shown by different investigators. Small and larger lymphocytes are thus believed to represent different stages on developmental pathways (vertical heterogeneity). The different populations of small thymocytes, on the other hand, are believed to represent different independent pathways (horizontal heterogeneity). There is clearly the possibility that functionally distinct sublines of peripheral T-cells are generated by separate developmental pathways in the thymus, and the functional properties of single thymic cell types (e.g. of the thymic suppressor cells) may accordingly correspond to the functional properties of their peripheral progeny.