The relative potency of porcine insulin and semisynthetic human insulin has been examined in humans using the euglycaemic glucose clamp technique. Three 90 minute consecutive infusions at 2.4, 4.8 and 7.2 units/hr were administered to 7 normal subjects. The rate of glucose infusion (mg/min, mean +/- SEM) required to maintain euglycaemia during the last 40 minutes of each insulin infusion for porcine (P) and human (H) insulin was not significantly different. However, when the glucose infusion rate was expressed as a function of the mean serum insulin level (mU/l) over the same period of time the values for P and H at 4.8 and 7.2 U/hr were not significantly different but H was greater than P (13.6 +/- 1.9, v 10.8 +/- 1.9, p less than .05) at 2.4 U/hr. Mean serum insulin levels did not differ significantly for human or porcine insulin infusion. This study confirms the similarity in potency between human and porcine insulin in the human. However, delivery at 2.4 U/hr, which would predominantly affect hepatic glucose output, suggests that semisynthetic human insulin is more potent than porcine insulin with respect to the liver.