The purpose of the study was the assessment of certain biological effects of semisynthetic human insulin compared with the presently used monocomponent preparations of porcine insulin made by the same producer. The study was carried out in a group of 10 healthy subjects (7 men and 3 women) twice: during a loading test with porcine insulin, and then during a similar test with human insulin. Both insulins were administered intravenously in doses of 0.075 units/kg body weight The physiological reactions associated with hypoglycaemia were noted, including subjective experiences and hormonal responses, among them those of glucagon, growth hormone, prolactin, adrenaline, noradrenaline and C-peptide. Semisynthetic human insulin administered intravenously was found to exert an identical hypoglycaemic effect as porcine insulin. However, it was observed that the action of porcine insulin was associated with more pronounced symptoms and more intense physiological reactions (tachycardia, body temperature fall) and a striking increase of prolactinaemia, in relation to semisynthetic human insulin.