AbstractThe novel existence of multiple binding sites for the potent nucleoside transporter Probe, [3H]nitrobenzylthioinosine, was identified in guinea pig platelet membranes and the binding characteristics compared to those of guinea pig erythrocyte membranes. Scatchard analysis of the binding in platelets reveled two high affinity binding sites with affinity constant (KD) of 0.94 ± 0.07 nM and 83 ± 13 nM with corresponding maximal binding capacities (Bmax) of 21 ± 7 and 110 ± 25 fmol/mg protein, respectively. In comparison, guinea pig erythrocyte membranes revealed a homogeneous population of the binding sites with KDof 0.17 ± 0.04 nM and a Bmaxvalue of 73 ± 11 fmol/mg protein. Biphasic semi‐log plots of the binding site heterogeneity in erythrocytes not reveled by Scatchard plots. Determination of the potencies of selected drugs in inhibiting the binding showed evidence of differential interacitons with the binding sites by various agents which may be exploited pharmacologically. © 1993 Wiley