Recombinant human interferon alpha A (code name: LBD-007) was subcutaneously administered to both sexes of Sprague-Dawley rats at the doses of 0, 3, 6 and 12 x 10(6) IU/kg of body weight six days per week for 6 months to evaluate the chronic toxicity. Male rats dosed at 12 x 10(6) IU/kg for 26 weeks showed a decrease of ketone body in the urine, and female rats dosed at 12 x 10(6) IU/kg for 26 weeks showed a decrease of serum alkaline phosphatase. Microscopic changes with remarkable incidence were nephropathy, renal mineralization and ultimobranchial cysts of the thyroid gland in all groups of both sexes of rats. These changes were not considered to be drug-related toxicity because they appeared to be minimal, occasionally mild in severity and did not have dose-dependency as well as clinical significance. In general, no drug-related change was noted in clinical findings, body weights, food consumption, water consumption, hematology, organ weights and necropsy findings. Based on the results, it is concluded that the estimated subcutaneous non-toxic dose of the recombinant human interferon alpha A (LBD-007) in rats would be 6 x 10(6) IU/kg.