Flow injection analysis, with chemiluminescence detection, is used to determine traces of copper(II) by means of the flavin mononucleotide–hydrogen peroxide–phosphate buffer system. This permits the determination of copper(II) more selectively than any other chemiluminescence system with a detection limit of 0.03 ng(20-μl sample injection) or 0.06 ng ml−1 (continuous sample flow). The linear range is 3 orders of magnitude, the sampling rate is 120 h−1, and the relative standard deviation is 3.1% for 1 ng Cu(II)(n=10). Iron(II) and chromium(III and VI), the strongest enhancer after copper(II), provide signals 2–3% of that for copper(II). Effect of surfactant micelles on the signal for copper is also discussed. The method is successfully applied to real samples.