We have investigated the role of endogenous ecotropic provirus in radiation induced and spontaneous hematopoietic neoplasms of the RFM/Un mouse. Our results indicate that somatically acquired provirus is integrated at novel sites in most reticulum cell sarcomas, and some radiation induced myeloid leukemias and thymic lymphomas. Detailed analysis of recombinant DNA clones of two somatically acquired proviruses in myeloid leukemia indicate that they are integrated into unique DNA regions that may be rearranged in independent tumors. These sequences may represent a cellular oncogene or other critical gene involved in specific neoplastic phenotype.