To determine if the more rapid absorption of subcutaneously administered human insulin, as compared with animal insulin, would result in an improved postprandial metabolic response, ten persons with type I diabetes mellitus were studied during a fixed meal. Plasma glucose and insulin concentrations were compared after subcutaneous injections of 0.2 U/kg body weight of regular biosynthetic human insulin or regular purified pork insulin in a double-blind randomized crossover trial. Meal glycemic excursions improved after the administration of biosynthetic human insulin, when compared with purified pork insulin (p less than 0.05 at 150, 180, and 210 minutes postprandially). Serum free immunoreactive insulin concentrations were significantly higher after injections of biosynthetic human insulin, and the rate of incremental rise during the first 30 minutes was also greater. Insulin antibody studies showed a strong negative correlation between peak insulin levels and the association constant for the high-affinity-binding insulin antibodies. We conclude that biosynthetic human insulin is more rapidly absorbed after subcutaneous injection than is purified pork insulin, a characteristic that results in improved postprandial metabolic control.