Six diabetic patients established on treatment with s.c. (n=3) or a continuous subcutaneous infusion (n=3) of porcine insulin took part in a trial of the continuous subcutaneous infusion of semisynthetic human insulin. The total daily insulin requirement was the same on porcine insulin (31.8±10.2) and human insulin treatment (30.6±12.8 U/day). One of the 3 patients who had been treated with CSII (porcine) showed a significant decrease in mean fasting plasma glucose level after commencement of CSII (human). The same was true regarding the change in HbA₁. The mean fasting plasma glucose was significantly decreased in 2 out of the 3 diabetics pretreated with s.c. porcine insulin after transfer to CSII (human). It seems likely, however, that the change in manner of insulin injection rather than the human insulin per se accounts for the observed effect. No significant change in insulin antibody titer was observed between before and after the CSII (human) treatment.