AbstractWe compared the reproducibility and temporal stability of ADP‐induced aggregation of platelet‐rich plasma anticoagulated with citrate or with D‐phenylalanyl‐L‐prolyl‐L‐arginyl‐chloromethyl ketone (PPACK), a thrombin inhibitor. Citrate‐ or PPACK‐platelet‐rich plasma (PRP) was stored at room temperature in capped plastic tubes or in plastic syringes from which all air was expelled. At intervals over 1‐5 hr after venipuncture, platelet aggregation was induced by 1‐1.5 μM ADP. Initially, the aggregation of PPACK‐PRP was nearly twice that of citrate‐PRP. The response of PPACK‐PRP stored in the syringe remained essentially constant over the interval of study, in contrast to the responses of the other samples which declined with time. The improved stability of the response obtained from samples anticoagulated with PPACK was due to the a citrate, since PRP containing both citrate and PPACK became less responsive over time a manner similar to PRP which contained only citrate. Anticoagulation with PPACK rather than citrate results in a more reproducible and stable aggregation response and more closely reproduces the in