The SV40 large T gene under the control of immunoglobulin enhancer induced hyperproliferation of multi-lineage hematopoiesis in transgenic mice. Hence the disease has been considered to be an appropriate experimental model for MDS-like myelodysplasia, sequential pathological changes in the development of the disease are introduced in the report. Huge splenomegaly was the major gross abnormality, which developed with 100% frequency; neither hepato-renal, nor other thymico-lymphatic involvement was common. During the progressive increase in splenic weight, extensive proliferation of multi-lineage hemopoiesis was prominent, although no differences were apparent in the cellular proportions of each hematopoietic element compared with normal spleens, either in flow-cytometric analysis using markers for each subset of hematopoietic elements, or in the histological findings. In the later phases of the disease, the proliferating cell type tended to shift to a variety of single to oligo-lineage hemopoiesis, but the majority of mice still showed the presence of multi-lineage hemopoiesis; histologically, such hemopoiesis was somewhat dysplastic, but had no apparent nature of leukemic infiltration. Several transplantation-assays essentially supported the low neoplastic potential of proliferating cells even in later phase. A long-term observation was made aiming to induce more frequent transition of this abnormal hemopoiesis into a single-lineage neoplasm by transplantation of pre-onset spleen cells, as well as bone-marrow cells from transgenic mice at an early phase of the disease, into lethally irradiated C57BL/6 mice. This trial resulted in a variety of neoplastic growths in the recipients; not only was myelodysplastic hypercellularity seen, but also, single-lineage hemopoietic malignancies, such as B-cell lymphomas/leukemias, histiocytic malignancies, and even myeloid leukemias. The transition from multi-lineage myelodysplasia into single lineage hemopoiesis at some frequency is reminiscent of myelodysplastic syndromes (MDS) in humans. Higher frequency of transition into lymphoid malignancies may be due partly to the immunoglobulin enhancer used as a promoter unit. The results that the SV40 large T antigen was expressed in every proliferating cells, there was no apparent increase in multi-CSFs activity; together with the results of the transplantation assays suggest that the hyperproliferation of the cells is directly induced by the expression of SV40 large T antigen in the hemopoietic cells themselves.