Bestatin has significant therapeutic activity (even following oral administration) for the treatment of metastatic disease, an activity which is limited by tumor burden. Therefore, the therapeutic potential of bestatin was examined in combination with chemotherapy to determine if there is additive activity for heavy tumor burdens. Bestatin significantly increased therapeutic activity and decreased the myelotoxicity of cyclophosphamide following a single injection of cyclophosphamide or split daily doses. In immune function studies, in tumor-bearing animals, bestatin increased the number of colony-forming units (granulocyte-macrophage) (CFU) and alveolar macrophage tumoricidal activity. However, when bestatin was combined with cyclophosphamide, which depressed bone marrow and macrophage activity, it did not show apparent augmentation of macrophage and N K cell activity, but did significantly increase bone marrow CFU activity. Thus, in combined chemoimmunotherapy, bestatin appears to enhance therapeutic activity by accelerating the recovery of hematopoiesis. We suggest, therefore, that a combination chemotherapy protocol, with oral bestatin, may facilitate myelorestoration following aggressive chemotherapy. The majority of biological response modifiers require parental administration; thus, the identification of an orally active, synthetic immunoaugmenting agent with a defined receptor is of particular interest.