The activity of semi-synthetic human insulin has been compared with porcine insulin in normal man using an euglycaemic glucose clamp at two different insulin infusion rates. In the two hour infusion insulin levels plateaued for both types of insulin at 44-48 mU/l (infusion rate 0.05 U kg body weight-1 h-1) and 22-24 mU/1 (0.02 U kg-1 h-1), giving identical metabolic clearance rates. The glucose delivery required to maintain euglycaemia in the second hour of insulin infusion was 13.9 +/- 2.1 g (mean +/- SEM) and 14.7 +/- 1.5 g (NS) at the lower dose for porcine and human insulins respectively, and 27.1 +/- 2.5 and 28.0 +/- 2.9 g (NS) at the higher dose. The potency ratio for human, compared with porcine, insulin was 1.06 +/- 0.12. No differences were seen in the time of onset of action of the insulins, serum half-life or distribution space. The responses of blood lactate, pyruvate, alanine, glycerol and 3-hydroxybutyrate were identical. No untoward reactions occurred. The activity and disposal of this semi-synthetic human insulin are indistinguishable from porcine insulin in normal euglycaemic man.