Grain count histograms from autoradiographs of [3H]uridine-labelled rat thoracic duct lymphocytes confirm the previously reported difference in uridine uptake between thymus- and marrow-derived small lymphocytes in rats. This difference is used in conjunction with autoradiography to demonstrate the specificity of cytotoxic antisera distinguishing thymus- and marrow-derived small lymphocytes.
Cytotoxic alloantisera detected a new polymorphic alloantigen system present on rat peripheral thymus-derived lymphocytes, but reduced or absent on thymocytes. At least three alleles or a complex genetic system are indicated. Rabbit antisera with specificity for rat marrow-derived small lymphocytes are described: these sera are not cytotoxic for haemolytic plaque-forming cells.