The effect of different doses of recombinant human tumor necrosis factor-alpha (rTNF-alpha) on serum levels of neopterin, beta-2-microglobulin and interferon-gamma (IFN-gamma) was investigated in tumor patients. Twelve patients with advanced malignant disease were treated and received single doses of either 1, 10, or 100 micrograms/m2 rTNF-alpha on days 0 and 7. Neopterin, beta-2-microglobulin and IFN-gamma serum levels were measured from day -2 to day 12 of the study. Application of rTNF-alpha leads to a marked and dose-dependent increase of neopterin and beta-2-microglobulin levels; no rTNF-alpha-dependent changes were observed after 1 microgram/m2, and maximum increments were seen in patients receiving 100 micrograms/m2. Serum levels of both parameters peaked after 2 days and returned to baseline values within 1 week. IFN-gamma levels were also elevated after application of rTNF-alpha. We failed, however, to demonstrate a clear correlation between the serum levels of IFN-gamma, beta-2-microglobulin, and neopterin because of the wide range of pre- and posttreatment levels of IFN-gamma. We conclude that neopterin and beta-2-microglobulin represent useful markers for monitoring biological response to treatment with rTNF-alpha.