The carboxamide-quinoline LS 2616 is a novel immunomodulator augmenting natural killer (NK) cell activity and T-lymphocyte related effector functions. To investigate the possible usefulness of LS 2616 in immunotherapy of tumors, the effect of the substance on growth and metastasis of the B16-F10 melanoma in syngeneic C57BL/6 mice was investigated. Treatment with LS 2616 from the time of s.c. inoculation of B16-F10 cells significantly reduced tumor take. Continuous treatment of mice with LS 2616 initiated 4 days prior to i.v. injection of tumor cells reduced the number of pulmonary metastases by 85%. When treatment with LS 2616 was started 4 days after i.v. injection of tumor cells, a time when established tumor foci were readily detectable in the lungs, a significant reduction in the number of pulmonary metastases resulted. LS 2616 significantly reduced the number of spontaneous pulmonary metastases developing from a B16-F10 tumor growing in the footpad. When treatment with LS 2616 was initiated after the establishment of grossly visible spontaneous pulmonary metastases, no significant effect on the number of metastases was found after 2 weeks of treatment. However, combined treatment with a dose of cyclophosphamide which in itself was ineffective resulted in a statistically significant 70% reduction in the number of remaining pulmonary metastases. Injection of antibodies to asialomonoganglioside which strongly reduce NK cell activity in various organs was used as a probe for the involvement of NK cells in the effects of LS 2616 on the B16-F10 tumor. The therapeutic efficiency of LS 2616 on tumor take when given from the time of s.c. inoculation, on the number of i.v. induced pulmonary metastases when treatment was started before tumor cell injection, as well as the spontaneous development of pulmonary metastases during exposure to the substance was abrogated by simultaneous injection with antibodies to asialomonoganglioside. In contrast, the beneficial effects of LS 2616 on already established i.v. produced or spontaneous pulmonary metastases were unaltered in mice made NK cell deficient by injection of anti-asialomonoganglioside antibodies. In conclusion, LS 2616 has potent antitumor activities mediated by NK cells as well as non-NK cell related defense mechanisms.