Recombinant human gamma interferon (Biogen) and vinblastine were administered in a phase I study. Side effects included fever and chills, nausea and vomiting, acute symptomatic hyponatremia, reversible myelosuppression, hepatitis, transient hypotension, congestive heart failure, renal insufficiency, and nonselective proteinuria. In most patients, additional host factors contributed to these toxic effects. Side effects occurred despite dose reduction; therefore, protocol accrual was prematurely closed. No correlation between serum concentrations and toxicity was noted. Median serum vinblastine concentration was 1.04 ng/ml; median serum interferon concentration was 17.3 IU/ml.