When 3-methylcholanthrene therapy was initiated 8 days after transplantation of the myeloma X5568 in CgH mice, retardation and even complete suppression of the transplant resulted. It did not appear that a general depression of somatic growth eouM account for this effect. Histological examination of the carcinogen-treated myeloma revealed peripheral necrosis, reduction in mitotic figures, without a striking infiltration of leukocytes or normal plasma cells. Dihydrotestosterone in conjunction

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... h 3-methylcholanthrene produced a significant increase in body weight of the host mice, but appeared to have no additive effect on suppression of the myeloma's growth. When therapy with this agent was delayed until 80 days after transplantation of the myeloma in the same strain of mice, complete regression of tumor still resulted occasionally, and the life of the tumor-bearing mice was generally prolonged. When 3-methyleholanthrene therapy was administered to C3H mice bearing sareomatous and/or carcinomatous tumors induced by this same carcinogen, tumor suppression and prolongation of survival were evident. The results are discussed in relationship to previously existing theories regarding mechanisms of action of the tumor-inhibitory polyeyelic hydrocarbons.