New polymeric electrodes responding to tiamulin hydrogen fumarate were constructed by incorporating tiamulin tetraphenylborate and tiamulin phosphotungstate ion-pair complexes into PVC membranes, with the latter giving the best linear and Nernstian response. The electrodes exhibited a linear response over the concentration range 10(-2)-10(-6) mol l(-1). The pH did not affect the response of the electrodes within the range 2-8. Direct potentiometry was used to assay tiamulin hydrogen fumarate in pharmaceutical formulations and in the bulk form. The results were compared with those obtained with non-aqueous titration for the antibiotic in the bulk form and with HPLC for commercial formulations.133457461