Several preclinical studies have found a poor correlation between the ex vivo platelet inhibitory potency and the in vivo antithrombotic efficacy of GPIIb/IIIa receptor antagonists. The present study was designed to examine the differential in vitro potencies of c7E3, MK-383, DMP-728, and SM-20302 in inhibiting ex vivo platelet aggregation under normocalcemic and hypocalcemic conditions. Human blood was collected in either trisodium citrate (0.37%) or PPACK (20 µg/mL). Platelet aggregation assays were performed in platelet-rich plasma from citrate-anticoagulated blood (cPRP) and PPACK-anticoagulated blood (pPRP) using ADP (20 µM) and TRAP (10 µM) as agonists in the presence of c7E3, MK-383, DMP-728, or SM-20302. The concentration of ionized calcium in cPRP was 16–19 times lower than that in pPRP. The IC50 of c7E3 for inhibiting ADP-induced platelet aggregation in cPRP (2.76 ± 0.11 µg/mL) was 1.6 times lower than that in pPRP (4.46 ± 0.48 µg/mL; P 50 for c7E3 for inhibiting TRAP-induced platelet aggregation in cPRP (4.52 ± 0.34 µg/mL) was 1.7 times lower than that in pPRP (7.69 ± 0.43 µg/mL; P 50 values, respectively, in cPRP as compared with pPRP. Chelation of ionized calcium in pPRP led to a progressive increase in platelet inhibition by all the antagonists. These results suggest that the observed in vitro inhibitory potency of a GPIIb/IIIa receptor antagonist is markedly enhanced when trisodium citrate is used as an anticoagulant to collect blood for ex vivo assay. These findings indicate that dosing regimens for GPIIb/IIIa receptor antagonists based on the platelet inhibition profile in citrate may provide misleading information with respect to their true in vivo antithrombotic efficacy.