The clinical characteristics of highly purified porcine NPH-insulin (Insulin Retard RI) were investigated, including absorption from the subcutaneous tissue, blood glucose-lowering effect, stability of mixtures of NPH and regular insulin and measurement of circulating porcine proinsulin and insulin antibodies in diabetes. The absorption of NPH-insulin followed first order kinetics. The half time was 6.9 +/- 2.6h, with an intraindividual coefficient of variation of 26% and an interindividual coefficient of variation of 55%. After 24 h 90% of the injected insulin had disappeared from the subcutaneous tissue. The plasma insulin concentration was maximal 4-5 h after the injection and 24 h after the injection it was not significantly higher than before the injection. The blood glucose-lowering effect was significant 2.5 h after subcutaneous injection of NPH-insulin and was maximal after 5.5 h. The blood glucose-lowering effect of a pre-prepared mixture of 70% NPH and 30% regular insulin was not significantly different from the effect of 70% NPH and 30% regular insulin injected separately, which indicates the stability of mixtures of NPH and Regular insulin. Porcine proinsulin disappeared from the serum of patients switched to treatment with highly purified porcine NPH insulin and the insulin antibody titer fell. It was concluded that insulin Retard RI has a well-defined, reproducible effect with a clinically useful time course.