A pilot study was undertaken in order to determine the feasibility and toxicity of rh-alpha-2b-interferon as maintenance therapy after allogeneic BMT for multiple myeloma. The study incorporated planned dose escalation of interferon in successive patient cohorts from an initial dose of 1 MU three times weekly to a target dose of 3 MU three times weekly. No clinical complications were observed in five patients receiving a dose of 1 Mu three times weekly. At a dose of 2 MU three times weekly, one of seven patients developed acute GVHD, which was fatal. At a dose of 3 MU three times weekly, four of five patients developed acute or chronic GVHD, and the study was therefore terminated at this point. We conclude that the use of interferon in myeloma patients early after allogeneic BMT is associated with a significant risk of GVHD, which is dose-related, and that the maximum tolerated dose in the early post-transplant period is 1-2 MU three times weekly.