: Mycophenolate mofetil (MMF) raises platelet counts in patients with primary immune thrombocytopenia. However, studies indicate that MMF inhibits collagen-induced platelet aggregation, potentially increasing bleeding risk following MMF therapy. The study evaluates the in-vitro effect of MMF on platelet function. Blood samples (n = 6) from healthy donors were incubated with vehicle, MMF or mycophenolic acid (MPA) at clinically relevant concentrations. Platelet aggregation was measured with flow cytometry and 96-well light transmission aggregometry (LTA). Using flow cytometry, we measured the expression of platelet CD49b, CD42b, CD42a, CD61 and CD41. Platelet activation was measured as the expression of P-selectin and the active form of the GPIIb/IIIa receptor following agonist stimulation. Agonists were: ADP, thrombin receptor-activating peptide, collagen, collagen-related peptide and U46619. The Platelet Function Analyzer-200 was used to measure global platelet function. MMF and MPA did not change platelet aggregation regardless of the agonist used. An exception was a significant, but minor decrease in collagen-induced platelet aggregation in samples with MMF (6 ± 3%, P = 0.02) and MPA (8 ± 4%, P = 0.01) compared with vehicle (22 ± 11%). However, this was not observed using the lesser sensitive LTA method. Compared with vehicle, MPA led to a significantly lower relative disposition of the surface collagen-receptor GPVI (7.8 ± 1.8 versus 8.8 ± 2.1 mean fluorescence intensity, P < 0.001). In all other platelet-related tests, neither MMF nor MPA showed any effect. In conclusion, MMF and MPA only had a minor effect on collagen-induced platelet aggregation, with MPA reducing the relative disposition of surface GPVI receptors.