The antiparasitic naphthylurea suramin (SUR) could be shown to exert immunomodulating and antimetastatic activity in BALB/c-mice. Regular intraperitoneal administration of SUR yielded significant weight gain of spleen and significant enhancement of peritoneal macrophage activity in chemiluminescence assays. The number of thymocytes per mg organ weight did not show evident differences in control and SUR treated groups; however, determinations of lymphocyte subsets revealed up-regulation of mature cells expressing helper/inducer (L3T4) or cytotoxic/suppressor (Lyt-2) phenotypes but down-regulation of immature cells presenting both L3T4/Lyt-2 antigens. Accordingly, administration of SUR obviously accelerated murine thymocyte maturation. Counts of BALB/c-mice peripheral blood lymphocytes (PBL) revealed evident (but statistically non-significant) increases after SUR administration. The determination of activated T-cells expressing interleukin (Il-2) receptors further proved that SUR induced a potent immunostimulation since these cells were significantly enhanced after treatment. To evaluate the antimetastatic activity of SUR, sarcoma L-1 cells were intravenously inoculated into BALB/c-mice. In this model system the number of experimental lung metastases significantly decreased, as compared to control mice which received injections of saline solution. Accordingly, SUR can be regarded as an immunomodulating and antimetastatic substance which seems to be promising for clinical trials in oncology.