JR14a, a selective and potent human Complement C3a receptor antagonist, which is assessed by inhibition of intracellular calcium release (IC50=10 nM) induced in human monocyte-derived macrophages by C3a, inhibition of β-hexosaminidase secretion (IC50=8 nM) from human LAD2 mast cells degranulated by C3a, and selectivity for human C3aR over C5aR.