The influences on host immunosuppression by treatment with cyclophosphamide (200 mg/kg), steroid (prednisolone, 12.0 mg/kg for seven doses or 235 mg/kg for one dose), and adult thymectomy on tumor growth were compared. Treatment with cyclophosphamide 24 hr prior to MOPC 104E tumor transplantation produced the greatest facilitation of tumor growth. The role of prednisolone in rendering the MOPC 104E cells more vulnerable to conventional chemotherapy was also investigated. The combination of prednisolone with melphalan added measurably to the cytotoxicity of the treatment and increased the percentage of disease-free survivors. The observed effects of prednisolone might have been due to the increase in the cycling of myeloma cells directly, or the drug may have facilitated growth of the myeloma by blocking host expansion of T-cell immunity. Alterations of the host by adult thymectomy and immunosuppression with cyclophosphamide or prednisolone led to growth facilitation of myeloma. The limited studies reported here point out the usefulness of facilitation of tumor growth to accomplish increased neoplastic cell kill and increased percentage of disease-free survivors. © 1983, American Association for Cancer Research. All rights reserved.