Syk Inhibitor II dihydrochloride dihydrate is a potent, high selective and ATP-competitive Syk inhibitor with an IC 50 of 41 nM. Syk Inhibitor II dihydrochloride dihydrate inhibits 5-HT release from RBL-cells with an IC 50 of 460 nM. Syk Inhibitor II dihydrochloride dihydrate shows less potent against other kinases and has anti-allergic effect.