By occupying specific surface receptors, adenosine and adenosine analogues modulate neutrophil functions; in particular, functional and biochemical studies have shown that A 1 adenosine receptors modulate chemotaxis in response to chemotactic peptides. Until now, the characteristics of the specific agonist binding and the visualization of A 1 receptors in human neutrophils have not been investigated. In the present study, we used the agonist [ 3 H] CHA for radioligand binding studies and a CHA-biotin XX probe in order to visualize the A 1 binding sites in human neutrophils, ultrastructurally, by conjugation with colloidal gold-streptavidin. [ 3 H] CHA bound A 1 adenosine receptors with selectivity and specificity, although with a low binding capacity. Scatchard analysis showed a Kd value of 1.4 ฯฎ 0.08 nM and a maximum density of binding sites of 7.1 ฯฎ 0.37 fmol/mg of proteins. The good affinity and selectivity of the CHA-biotin XX probe for A 1 adenosine receptors allowed us to visualize them, after conjugation with colloidal gold-streptavidin, as electron-dense gold particles on the neutrophil surface and inside the cell. The internalization of the ligand-receptor complex was followed in a controlled temperature system, and occurred through a receptormediated pathway. The kinetics of the intracellular trafficking was fast, taking less than 5 min. These data suggest that the CHA-biotin XX-streptavidin-gold complex is a useful marker for the specific labelling of A 1 binding sites and to follow the intracellular trafficking of these receptors.