Immunotherapy with the immunomodulating thymic humoral factor-gamma 2 (THF-gamma 2) octapeptide, combined with 1,3-bis(2-chloroethyl)-1-nitrosourea (BCNU) chemotherapy, will be used for enhancing host immune response to arrest pulmonary metastases of a B16-F10.9 melanoma tumor. In this experimental model of pulmonary metastasis, the highly metastatic B16-F10.9 melanoma tumor cells (2 x 10(5)) were inoculated into the footpad of mice to form a primary tumor. The tumor-bearing leg was surgically removed on reaching the size of 5.5 mm, which resulted in the appearance of metastases in the lungs of the animals. After tumor excision, mice were treated intraperitoneally with a single dose of BCNU (20 or 35 mg/kg) followed by a series of intraperitoneal THF-gamma 2 injections (1 microgram/0.5 ml/injection). Relative to untreated mice and those receiving chemotherapy alone, the antitumor action of the combined THF-gamma 2 chemoimmunotherapy protocol was significantly augmented according to the following in vivo parameters: (a) decreased postsurgical spontaneous metastatic burden; (b) prolonged survival time; (c) increased resistance to tumor cell challenge; and (d) massive infiltration of lymphocytes, polymorphonuclear cells, and macrophages in the lung tissue. The THF-gamma 2 immunotherapy also prevented a decrease in lymphocyte reactivity, otherwise induced by the tumor/BCNU chemotherapy. THF-gamma 2 immunotherapy resulted in restoration of the response to Lipopolysaccharide mitogenic stimulation and the allogeneic response. Our data suggest that postoperative THF-gamma 2 immunotherapy could be a valuable adjunct to anticancer chemotherapy as a treatment for metastatic arrest of melanoma tumor.