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1. Normal human blood platelets in stirred plasma were incubated with [¹⁴C]ADP for 10-360 sec and the aggregation responses were correlated with platelet bound radioactivity, the platelets being separated from the plasma within 25 sec of the end of the experiment.

2. The platelet aggregation response, measured as a change in light transmittance through platelet-rich plasma, was related to the plasma [¹⁴C]ADP concentration and linearly related to the log platelet bound [¹⁴C]ADP 60-120 sec after addition of nucleotide to plasma.

3. Thin layer chromatography of the platelet bound radioactivity showed that 78-90% was unmetabolized ADP, the remainder being AMP. Plasma radioactivity consisted of ADP, AMP and adenosine. There was no detectable radioactive cyclic AMP in either platelets or plasma.

4. Further accumulation of radioactivity occurred after 180 sec but was not related to the aggregation response.

5. Prostaglandin E₁ inhibited aggregation and platelet [¹⁴C]ADP accumulation when added to platelet-rich plasma 60 sec before [¹⁴C]ADP. There was a significant correlation between inhibition of aggregation and inhibition of [¹⁴C]ADP accumulation.

6. Prostaglandin E₁ also reversed ADP aggregation when added to platelet-rich plasma after the nucleotide, with an accompanying decrease in platelet bound [¹⁴C]ADP.

7. It is concluded that ADP induces platelet aggregation by binding to specific receptors probably located on the plasma membrane, and that prostaglandin E₁ inhibits this effect by interfering with the ADP binding.