The antitumour effect of recombinant human interferon (rh-IFN) alpha-2b was studied in 22 patients with advanced multiple myeloma (MM). Nine of 14 evaluable cases were refractory to cytostatic therapy; five were in relapse. rh-IFN was administered s.c. three times per week, in escalating doses starting with 2 x 10(6) IU m-2 and if possible up to 15 x 10(6) IU m-2. Two patients (one refractory, one relapsing) showed a partial response, defined as a 50% reduction of the serum M-component. Three further patients had a minor, significant but short-lived response. Subjective side-effects grade 1-2 were noted during rh-IFN therapy in all patients. In three cases thrombocytopenia necessitating platelet transfusions occurred. Although a fraction of patients with advanced MM obviously respond to rh-IFN, this type of therapy may be more effective, alone or in addition to chemotherapy, in patients with a low tumour cell burden.