Easily reproducible animal models that allow for study of the biology of chronic lymphocytic leukemia (CLL) and to test new therapeutic agents have been very difficult to establish. We have developed a novel transplantable xenograft murine model of CLL by engrafting the CLL cell line MEC1 into Rag2^−/−γ_c^−/− mice. These mice lack B, T, and natural killer (NK) cells, and, in contrast to nude mice that retain NK cells, appear to be optimal recipient for MEC1 cells, which were successfully transplanted through either subcutaneous or intravenous routes. The result is a novel in vivo model that has systemic involvement, develops very rapidly, allows the measurement of tumor burden, and has 100% engraftment efficiency. This model closely resembles aggressive human CLL and could be very useful for evaluating both the biologic basis of CLL growth and dissemination as well as the efficacy of new therapeutic agents.