Biosynthetic human insulin (BHI) produced by recombinant DNA technology has been proven to be identical to pancreatic human insulin. It differs from pork insulin in one carboxy-terminal amino acid of the B-chain, where alanine substitutes for threonine. This leads to a higher hydrophilicity and usually lowers the tendency of insulin molecules to aggregate. To study its biologic effect in man, the Gerritzen test was applied. BHI regular (10 IU) or purified pork insulin (PPI) (10 IU) was injected subcutaneously to evaluate the insulin action profile. The time course of plasma glucose levels under both BHI and PPI did not significantly. BHI seems to lower the blood glucose somewhat faster during the first 30 min and, during the late phase, somewhat less than pork insulin. In general, for both BHI and PPI, the onset of action occurs within 15 and 30 min; after 60 min, plasma glucose levels drop by 44.3% for BHI and by 40% for PPI. The plasma glucose minimum was reached about 2 h after injection; therefore, the slope of the plasma glucose fall after 60 min was only marginal. The minimum levels were steady over about 1.75 h. At 2.5 h postinjection plasma glucose rose gradually and reached starting levels after about 7 h. BHI was as well tolerated in the intradermal skin testing as were pork insulin and a placebo solution.