The serum insulin, fractional absorption, serum human C-peptide, and plasma glucose responses of normal fasting subjects were compared after the subcutaneous and intravenous administration of human insulin (recombinant DNA) and Novo Actrapid insulin. While no statistical difference at each time point was observed between the two insulins, for each time point after 0.05 and 0.1 U/kg s.c., the 0-6 h area under curve (AUC) after the 0.05 U/kg dose was greater for human insulin than for pork insulin. There was no difference in the 0-6 h AUC after the 0.1 U/kg s.c. dose. The serum human C-peptide responses to the two insulins were virtually identical. With the 0.05 U/kg s.c. dose, hypoglycemic effect of human insulin was greater than for Actrapid. This difference did not occur after 0.1 U/kg s.c. Following intravenous administration using 0.05 U/kg, the serum IRI, serum C-peptide, and glucose responses were the same. These data indicate only slight differences between human insulin and Actrapid insulin.