We studied, by means of quantitative histopathological methods, the changes that took place in B16 melanoma implanted into C57BL/6J mice after combination treatment of hyperthermia, melphalan, and 1-(2-chloroethyl)-3-cyclohexyl-1-nitrosourea (CCNU). The studies were made on animals sacrificed on days 11, 18, and 25 after implantation. Tumors in treated animals showed a progressive delay in growth, a noticeably reduced number of metastases, and diminution of proliferative capacity. However, this treatment did not affect necrosis, stroma, capsule, or cell infiltration. Ultrastructurally, signs of cell damage were constantly present.