The human adenocarcinoma cell-line HT29 was used as a model to investigate the binding properties of a new antagonist radioligand of the imidazoline series, [3H]RX821002. All aspects of [3H]RX821002 binding conclusively prove that this radioligand is a valuable tool for labelling alpha 2A-adrenoceptors. [3H]RX821002 binding was very rapid and reversible. Computer-assisted analysis of kinetic data revealed association and dissociation time courses consistent with a simple bimolecular reaction. Saturation isotherms indicated that [3H]RX821002 labeled with high affinity a single population of non-interacting sites displaying a KD of 1.7 +/- 0.1 nM. Adrenoceptor agonists and antagonists inhibited [3H]RX821002 and [3H]yohimbine binding with a strictly similar rank order of potency which is characteristic of alpha 2A-adrenoceptors. The binding parameters of [3H]RX821002 were compared with those of other commercially available [3H]antagonists, [3H]yohimbine and [3H]idazoxan. Analysis of the saturation isotherms for the three radioligands showed that (1) [3H]RX821002 was the radioligand exhibiting the lower percentage of non-specific binding and the better affinity, (2) the Bmax of [3H]RX821002 was significantly higher than that of [3H]yohimbine. The difference in Bmax was not due to better labelling of one of the two affinity states of the receptor but was greatly reduced in glycylglycine buffer, suggesting that, in Tris-Mg2+ buffer, [3H]yohimbine does not label the entire alpha 2-adrenoceptor population.