An intrathymic (i.t.) injection assay on B10.Thy-1 congenic mice was used to demonstrate that thymic prelymphoma cells developed first within mouse thymus 4 to 8 days after split-dose irradiation and were present in more than 63% of the test donor mice thymuses examined 21 and 31 days after irradiation. For the characterization of these thymic prelymphoma cells, thymocytes from B10.Thy-1.1 mice sampled 1 mo after irradiation were stained with J11d mAb and mAb against TL-2 (thymus-leukemia) antigen which is not expressed on normal thymocytes of the B10.Thy 1.2 and B10.Thy 1.1 strains but does appear on thymocytes of split-dose irradiated mice. These cells were sorted into subpopulations, samples of which were injected into recipient thymuses to determine which subpopulations contained thymic prelymphoma cells. Results showed that the prelymphoma cells were located in the J11d+TL-2+ cells. These prelymphoma cells were further characterized phenotypically as to their expression of the CD4 and CD8 antigens, which demonstrated that the thymic prelymphoma cells were present in the CD4-CD8- and CD4-CD8+ thymocyte subpopulations mainly and in the CD4+CD8+ subpopulation. The experiments on i.t. injection of a graded quantity of TL-2+ thymocytes from individual mice suggest that not all TL-2+ cells undergo neoplastic initiation and that prelymphoma cells may develop infrequently from one or more TL-2+ cells by genetic or epigenetic changes.