Three hundred thirty-five previously untreated patients with multiple myeloma in clinical stages II and III entered a randomized trial comparing intermittent oral melphalan and prednisone (MP) therapy (n = 171) with MP in combination with natural (leukocyte-derived) alpha- interferon (MP/IFN) (n = 164). The treatment groups were comparable with regard to major prognostic factors. The response frequency was 42% in the MP group and 68% in the MP/IFN group (P < .0001). Eighty-five percent of IgA myelomas and 71% of Bence-Jones myelomas responded to MP/IFN compared with 48% and 27%, respectively, to MP treatment (P = .001). There was no difference in the overall survival between the two treatment groups. However, the survival of 72 patients with IgA or Bence-Jones myeloma randomized to receive MP/IFN was significantly longer (median 32 months) than that of 71 patients treated with MP (median 17 months) (p < .05). No statistically significant difference in response frequency (60% v 46%) or survival was found for patients with IgG myeloma. Hematologic toxicity, WHO grades III and IV, was higher in the MP/IFN group (48%) than in the MP group (33%) (P <.05) during the induction treatment period. Flulike syndrome was observed in 68% of patients receiving MP/IFN. The results show that MP/IFN is a well-tolerated treatment regimen, superior to MP for remission induction, and it improves significantly the overall survival for patients with IgA and Bence-Jones myelomas.