A phase Ia-Ib study was undertaken to treat melanoma patients with a constant dose of the anti-GD3 monoclonal antibody, R24, in combination with increasing dose levels of recombinant interferon-alpha (rHuIFN alpha-2a). Fifteen patients were treated on days 1-5 and 8-12 with a continuous 6-h i.v. infusion of R24 (8 mg/m2) and escalating i.m. doses of rHuIFN alpha-2a. Peripheral blood lymphocytes were obtained at multiple times before and during treatment and monitored for changes in lymphocyte subpopulations and changes in natural killer and antibody-dependent cellular toxicity functional activity. There were no consistent changes in most immune parameters; however, there was a decrease from pretreatment levels in the suppressor T cell (CD8+, CD11b+) subset and a dose-dependent decrease in the helper/inducer (CD4+, Leu-8+) T cell subset. The peak serum concentration of R24 was reached on day 5 of the study and was 9.4 micrograms/ml. During the second week of treatment, peak serum levels of R24 fell to less than 4 micrograms/ml. This finding was related to the development of human antimouse antibody, which would be detected as early as day 8 of the study. Binding of mouse Ig (R24) within the tumor bed was observed in 5 of 12 biopsy specimens. The maximal tolerated dose of the combination was dose level IV, in which patients received 8 mg/m2 of R24 and 50 x 10(6) units of rHuIFN alpha-2a on days 1-5 and 8-12 of treatment.