Spleen cell populations from mice treated with Adriamycin or daunorubicin were found to develop a greater PHA-stimulated clonal response than spleen cells from untreated or cyclophosphamide-treated animals. Daunorubicin, in addition, stimulated an unusual monocytic macrophage-like colony. The results are consistent with the possibility that the concentration of nonspecific T-lymphocyte progenitor and/or accessory cells in the spleen are increased consequent to drug treatment.