Radioimmunoassay (RIA) has proved to be a highly successful analytical method, but is not without disadvantages. The obligatory physical seperation of antibody-bound and free fractions of labelled antigen or hapten is a practical inconvenience and potential source of imprecision. Radiolabelled materials are often costly, may have limited shelf-life, present a radiation hazard, and necessitate expensive counting equipment. Non-isotopic labelling techniques which can obviate many or all of these shortcomings have accordingly received much attention, which has been largely focussed on enzyme immunoassay methods [ 1,2]. However, fluorescent-labelling also merits consideration when the extreme sensitivity of RIA is not called for. Fluoroimmunoassay methods based on detection of the extent of antibody binding of fluorescent-labelfed hapten in unseparated immunoassay incubation mixtures by measurements of fluorescence polarisation (polarisation fluoroimmunoassay) or fluorescence quenching (quenching fluoroimmunoassay) have recently been applied to assay of gentamicin in serum [3,4]. The present report describes the preparation of a fluorescent derivative of thyroxine (T4) whose fluorescence is enhanced when bound by anti-T4 serum, and illustratas in principle the exploitation of the effect in an ‘enhancement fluoroimmunoassay’ of T+