The efficacy and safety of transferring diabetic patients being treated with animal insulin to recombinant deoxyribonucleic acid human insulin (biosynthetic human insulin) was evaluated in 481 patients. Paired initial and follow-up random plasma glucose and hemoglobin Alc levels were available for 330 individuals. One year or longer after transfer to human insulin, the patients' morning and evening insulin doses remained virtually unchanged. After transfer, there were significant decreases in the mean levels of random plasma glucose, in glycosylated hemoglobin Alc, and in serum insulin antibody titers. These results indicate that patients who are being treated with animal-source insulins (ie, beef and/or pork) can be safely and effectively transferred to treatment with biosynthetic human insulin.