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Introduction: Little is known about the role of P2Y12 oligomerization in receptor function and whether P2Y12 receptor mutations associated with human bleeding disorders may be explained by alterations in oligomerization. Objectives: 1) To determine whether P2Y12 homo- and hetero-oligomers are constitutive or dynamically regulated. 2) To explore whether P2Y12 mutants R256Q and R265W (previously detected in patients with abnormal bleeding, but with unaltered ADP binding) have different oligomerization affinities or kinetics and determine whether differences in P2Y12 oligomerization explain the functional defects. Methods: We employed a Venus-based Bimolecular Fluorescence Complementation (BiFC) approach in HEK293T cells transiently co-expressing P2Y12 or its mutant forms (R256Q or R265W) tagged with either the N-terminal (P2Y12-VN) or C terminal fragment (P2Y12-VC) of Venus, to characterize their homomeric interactions, in live cells using confocal microscopy and quantitative flow cytometry assays. Results: Agonist-independent formation of P2Y12 receptor homo-oligomers were detected on cell membranes. Time lapse imaging showed movement of P2Y12 receptor pairs from the endoplamic reticulum and Golgi network to the plasma membrane, suggesting that they are constitutive and required for export. Co-expression of P2Y12-VN with increasing amounts of P2Y12-VC demonstrated a dose-dependent increase in the fluorescence intensity of Venus, and reached saturation at a ratio of 1:3. Interestingly, the fluorescence intensities of homomeric P2Y12-R256Q-VN and R256Q-VC and, separately, P2Y12-R265W-VN and P2Y12R265W-VC were almost 4 times stronger than that of the wild type receptor as quantified in flow cytometry-based BiFC. Similar results were obtained in confocal microscopy. This suggests that these P2Y12 mutants form an increased number of dimers or oligomers with increased self-affinities. Conclusion: We demonstrate that P2Y12 forms constitutive homo-oligomers. Two mutations associated with bleeding disorders in patients have altered receptor-receptor interactions. Future investigation will explore the effect of mutations and receptor oligomers on G protein coupling and receptor: G protein stoichiometry.