A clinical transfer trial was conducted to ascertain whether semisynthetic human NPH insulin has a full 24-hour duration of therapeutic effect comparable to that of NPH insulin from animal sources. Diabetic patients requiring insulin and stabilized on a once-a-day (QD) regimen of animal NPH insulin were enrolled and entered a two-week run-in period during which the constancy of their insulin requirements and the stability of their glycemic control were assessed. At the end of the run-in phase, baseline measurements were made of fasting blood glucose (FBG), hemoglobin A1 C, C-peptide, and insulin antibodies. Patients then were transferred to a QD regimen of semisynthetic human NPH insulin (Novolin N) in the same dose as the animal insulin. Glycemic control was reassessed after 1, 4, and 8 weeks of therapy, and a global assessment of overall glycemic control was made at the conclusion of the study. Efficacy variables were analyzed for 39 patients. Most had non-insulin-dependent diabetes mellitus (NIDDM) and most were transferred from mixed beef/pork insulin. Six (15%) patients required significant adjustments in insulin dose or regimen; the remaining 85% completed the eight weeks of treatment with minimal changes in insulin dose. Mean values for FBG and hemoglobin A1 C did not change significantly between baseline and the end of the study. The only statistically significant change was an increase in mean body weight (P less than or equal to 0.01). Results of the investigators' global assessments showed that 74% of the patients had unchanged or improved control of glycemia after transferring to semisynthetic human NPH insulin. The average frequency of hypoglycemic events was not significantly changed.(ABSTRACT TRUNCATED AT 250 WORDS)