Acute oral, intramuscluar, and intravenous toxicity studies of recombinant human interferon $alpha$2$alpha$(rhIFN $alpha$2$alpha$) were performed in Sprague-Dawley (SD) rats. SD rats were administered with doses of 31.25, 62.5, 125, 250 and 500 MIU/kg, respectively, and clinical signs, mortality and body weight changes were observed for 2 weeks. In all animals administered with rhIFN $alpha$2$alpha$, there was neither dead animals nor significant changes of body weights. In addition, no differences were found between control and treated groups in clinical signs and autopsy findings. Therefore, $LD_{50}$ of rhIFN $alpha$2$alpha$ was considered to be higher than 500 MIU/kg in SD rats.