Human insulin synthesised by recombinant DNA technology was compared with highly purified porcine insulin in healthy men. Intracutaneous injection over a wide range of concentrations of both insulins into five subjects gave rise to no local reactions over a 48 h period. The glycaemic response to standard subcutaneous injection at high and low dose levels was measured with both insulins in each of six men. Plasma glucose decrement with the two insulins was similar but human insulin was perhaps slightly more potent than porcine insulin at the low dose, and slightly less so at the high. The glycaemic response to the isulins, each infused intravenously at high and low concentrations for 1 h in a further six subjects, showed a similar trend. Depression of glycaemia with human insulin slightly exceeded that with porcine insulin at the low concentration infusion and fell slightly short of it at the high. Genetically synthesised human insulin seems to be safe and effective in man. Its dose-response relationship may differ from that of porcine insulin.