Multiple myeloma is a plasma cell neoplasm with an extremely variable clinical course. Animal models are needed to better understand its pathophysiology and for preclinical testing of potential therapeutic agents. Hematopoietic cells expressing the hypermorphic Rad50(s) allele show hematopoietic failure, which can be mitigated by the lack of a transcription factor, Mef/Elf4. However, we find that 70% of Mef(-/-)Rad50(s/s) mice die from multiple myeloma or other plasma cell neoplasms. These mice initially show an abnormal plasma cell proliferation and monoclonal protein production, and then develop anemia and a decreased bone mineral density. Tumor cells can be serially transplanted and according to array CGH and whole exome sequencing, the pathogenesis of plasma cell neoplasms in these mice is not linked to activation of a specific oncogene, or inactivation of a specific tumor suppressor. This model recapitulates the systemic manifestations of human plasma cell neoplasms, and implicates cooperativity between the Rad50(s) and Mef/Elf4 pathways in initiating myelomagenic mutations that promote plasma cell transformation.