The development of the leukemia-lymphoma complex was studied in AKR/J (H-2k) mice using flow cytometry and staining with acridine orange. Investigation of cytokinetics and of cellular RNA content showed that during the neonatal period all mice had a significant increase of S phase cells in the thymus, bone marrow, lymph nodes and spleen reflecting extramedullary hematopoiesis. Concomitantly, G0/G1 cells were significantly reduced in the thymus, lymph nodes and spleen when compared to 6-week old mice of the same strain. No changes in the cell cycle or in RNA content were observed until 10 months of age in congeneic AKR (H-2b) mice, which do not develop leukemia during the first year of life. In leukemia-prone AKR/J (H-2k) mice, however, it was shown that the first appearance of a leukemic process may be recognized in the thymus by a significant increase of cells in G1 phase of the cell cycle which have a high RNA content. These changes were first seen at 5 months of age before the increased expression of MuLV antigen signals preleukemic alterations at 6 months of age and long before morphological changes appear (8-10 months). Furthermore, at 6 months these mice showed a significant elevation of cells in S phase which always appeared initially in the thymus. By 10 months of age, when the mice were overtly leukemic, these changes had progressed in all lymphoid organs and in the peripheral blood. At the same time a unique population of cells was observed that was characterized by cells in S and G2M with very low RNA content. The method used is applicable to further analysis of the precise locus of development of leukemia in the thymus of AKR/J (H-2k) mice, analysis of the nature of the earliest malignant cells, and investigation of the influence of viruses in the pathogenesis of AKR leukemia.