PAR4 antagonist 2 (Compound 31) is a protease-activated receptor 4 (PAR4) antagonist with IC50 values of 95 nM for human PAR4 and 367 nM for mouse PAR4. It inhibits the activation of PAR4 through cleavage by the natural protease thrombin but has no effect on the synthetic PAR4 agonist peptide AYPGKF.