The miscibility of human semisynthetic regular and lente insulins (Actrapid human/Monotard human) and human biosynthetic regular and NPH insulins (Humulin regular/Humulin NPH) was studied in vitro and in 16 insulin-dependent diabetic patients. In vitro a decrease of regular insulin was found immediately after mixture with semisynthetic lente insulin in proportions 1:4, 1:3, 1:2, 1:1, and 2:1. This was not found after mixture with human biosynthetic regular and NPH insulins. Free insulin and blood glucose were determined over 8 h after the morning injection of regular/lente (n = 9) or regular/NPH (n = 7) insulins in proportions used by the patients (approximately 1:2) mixed in one syringe or in separate syringes. Mixing the regular/lente preparations in one syringe immediately before injection resulted in a significant loss of the early rise in free insulin (P less than .01), with a tendency to a higher free insulin in the afternoon. A corresponding change in the blood glucose profile was seen. No changes were observed in free insulin or blood glucose after mixing regular and NPH preparations compared with separate injections. We conclude that mixing human semisynthetic regular and lente insulins (Actrapid human/Monotard human) instantly results in a decrease of regular insulin, probably due to formation of a longer-acting preparation, whereas no such changes occur with human biosynthetic regular and NPH insulins (Humulin regular/Humulin NPH).