Stromal interaction molecule 1 (STIM 1) has been described to communicate the content of the stores to plasma membrane channels during store operated calcium entry (SOCE) in a number of cells, including human platelets (Ong et al. 2007). STIM1 EF-hand domain detects intraluminal calcium levels and allows STIM multimerization into puncta in close apposition to calcium channels in the plasma membrane, such as canonical TRP or Orai proteins. (Ong et al. 2007; Muik et al 2009) STIM 1 can be regulated by Ser and Pro phosphorylation (Manji et al. 2000; Smyth et al. 2008). Hence we have investigated whether STIM1 could also be regulated by phosphorylation in Tyr residues during SOCE activation. Blood samples from healthy donors were obtained according to the Declaration of Helsinki and platelets were isolated as previously described (Rosado and Sage 2000). Fura 2-loaded platelets were stimulated with thapsigargin (TG, 200 nM) and calcium mobilization was registered using the rapid kinetics Stop flow system. Subsequently, platelets were stimulated with TG (200 nM) by using a Quench flow system and fixed by mixing with RIPA at different stimulation times. STIM1 was immunoprecipitated from platelet lysates using 2 µg/mL anti-GOK/STIM1 antibody (overnight at 4°C). SDS-PAGE and Western blotting using a specific anti-phosphotyrosine antibody (4G10) revealed that maximal STIM1 tyrosine phosphorylation was detected 2.5 s after addition of TG. STIM1 maximal phosphorylation time-point was not modified upon removal of extracellular or both extra- and intracellular calcium with either EGTA (100 µM) alone or in combination with dimethyl BAPTA (loading with dimethyl BAPTA-AM 10 µM for 30 min). Finally, platelets suspended in absence or presence of calcium, were treated with the dual Src/Lyn inhibitor, Bosutinib (1,5 μM) for 30 min at 37°C, resulting in the inhibition of STIM1 tyrosine phosphorylation, an effect that was found to be greater in the presence of calcium, which indicate that Src family members, such as pp60src, might be involved in STIM1 tyrosine phosphorylation during the activation of SOCE. TG-evoked SOCE resulted significantly reduced in presence of bosutinib, thus suggesting that STIM1 tyrosine phosphorylation plays a functional role in SOCE in human platelets.