Ecotropic virus-producing cells in the bone marrow of the leukemia-prone AKR strain of mice were significantly enriched by fractionation on discontinuous density gradients of Percoll and were found in a low-density population of cells comprised predominantly of medium to large blast cells. The high ecotropic virus-producing low-density bone marrow cell population was also found to be significantly enriched in pluripotent stem cells, prothymocytes, and stromal elements. During the period of time defined by a window for successful leukemosuppressive immunotherapy of AKR mice, virus-producing cells were exclusively detected in this fraction of bone marrow cells, implicating the functional classes of cells coenriched in this fraction as both potential targets of anti-viral immunotherapy and responsible for the seeding of the spleen and thymus with infectious ecotropic virus.