A chemiluminescence (CL) flow system for determination of thyroxine (Thy) is presented. It is based on the catalytic effect of cobalt(II) on the CL reaction between luminol and hydrogen peroxide. The iodinated chemical structure of Thy causes a heavy atom effect. The luminol CL signals show significant quenching by Thy. The calibration graph for Thy is linear for 15-70 mug ml(-1) and the 3sigma detection limits are 27 mug ml(-1) for D-Thy and 23 mug ml(-1) for L-Thy. (C) 2002 Elsevier Science B.V. All rights reserved.