Adenosine diphosphate (ADP) caused rapid and reversible aggregation of turkey thrombocytes, as determined by optical and microscopical examination. To investigate the binding of ADP to these cells, turkey thrombocyte-rich plasma was incubated with ADP-8-14C or ADP-3-3H and with human serum albumin labeled with 125I. The thrombocytes were rapidly separated by centrifugation through silicone oil. From radioactivity determination of plasma and thrombocyte pellets the uptake of ADP radioactivity by thrombocytes was calculated. Scatchard analysis of concentration-dependent binding yielded a hyperbolic plot which indicates that turkey thrombocytes similarly to human platelets have more than one type of binding site for ADP.