We have developed and validated an automated kinetic method for angiotensin-converting enzyme (EC 3.4.15.1) on the Olli C + D analyzer, modified from that of Ronca-Testoni (Clin Chem 1983;29:1093-6) with N-[3-(2-furyl)-acryloyl]-L-phenylalanylglycylglycine used as substrate. We have determined appropriate reaction conditions for the assay, verified the principal analytical reliability criteria (repeatability, reproducibility, sensitivity), and established normal reference intervals (mean +/- SD) for the enzyme's activity, using serum of normal adults (100 +/- 35 U/L, n = 150), newborns (130 +/- 27 U/L, n = 10), women taking oral contraceptives (103 +/- 30 U/L, n = 10), smokers (109 +/- 38 U/L, n = 27), and patients with sarcoidosis (220 +/- 48 U/L, n = 15).