Abstract: Pharmacological characterization is described for a human imidazoline binding site (I-site) labeled by [3H]clonidine using standard autoradiographic method. Under conditions that mask a2-adrenergic sites, only a single high affinity site was observed in human caudate and blood platelet sections. Affinity constants (Ki) were highly correlated between the two tissues (r 5 0.90, P 5 0.0003). This site is dissimilar to classical I1 and I2 sites, even though both tissues possess abundant I1 and I2 sites by filtration binding methods. It is suggested that the isotonic buffer conditions inherent to the procedure alter drug affinities to the classical I1 site.