The pharmacodynamics and biologic activities of recombinant human interferon-β (rHuIFN-β) derived from Chinese hamster ovary (CHO) cells were examined during long-term therapy in 7 melanoma patients. The CHO-derived rHuIFN-β was given s.c. in a dose of 3×10⁶ U three times per week for 24 weeks. Serum levels of IFN could not be detected before and 48 h after the s.c. injections. 2′-5′-O1igoadenylate synthetase (2-5 OAS), β₂-microglobulin, and neopterin levels increased significantly 48 h after application, with a maximum after 96 h. Subsequently, the values decreased and remained only slightly elevated during the long-term therapy. Natural killer (NK) cell activity increased in the first 96 h significantly and fell below pretreatment values after 4 weeks. The decrease of biologic response could not be attributed to the occurrence of anti-IFN-β antibodies because only 2 of the 7 patients developed neutralizing antibodies after 16 and 24 weeks of treatment, respectively. This trial confirms the biologic potency of CHO-derived rHuIFN-β. However, the selected parameters demonstrate that immunostimulation is only possible over a short treatment period.