Normal and thrombasthenic platelet membranes are able to specifically bind 14C-ADP (12). According to the concentration of ADP present in the medium, ‘high affinity binding sites’ (Ka = 0.5 × 106M-1) and ‘low affinity-binding sites’ (Ka= 0.05 × 106 M-1) can be recognized. In the present study, dissociation of ADP from the ‘high affinity binding sites’ was measured with six normal and with three thrombasthenic platelet membrane preparations. A 1:100 dilution was used to dissociate the membrane-bound 14C-ADP and the kinetics of the dissociation was determined. The same profile of dissociation (with T 1/2 = 4–10 min at 37 °C) was observed using normal or thrombasthenic membranes. With both preparations, the rate of dissociation was increased (up to T 1/2 = 1 – 3 min) when unlabelled ADP (at concentration higher than 10-5M) was added in the diluting medium. The results confirm the presence of normal ADP binding sites on thrombasthenic platelet membrane and possibly suggest the existence of cooperative interactions between the sites on normal as well as on thromb