Abstract: This study describes the development of a competitive immunoassay for triiodothyronine (T3) comparing a flow injection and a modified sequential injection technique. The detection is carried out by chemiluminescence using an acridinium ester as a label for the antigen. Both immunoreaction and chemiluminescence reaction take place in a flow-cell packed with immobilized antibodies. The sequential injection technique was applied with and without flow-reversal. The detection limits for all types of assays were in the low fmol range (0.4 ng ml-1), whereby, with the flow-reversal mode, a slight improvement of the detection limit was observed. The relative standard deviation for the flow injection-and the sequential-injection mode were 4.4-7.9% and 2.2-5.5%, respectively. The total assay time was 7-9 min, depending on the mode.