– MASP3 is a member of the MASPs involved in mannan-binding lectin (MBL) complement pathway. The MBL pathway is initiated by the binding of MBL to specific carbohydrate structures found on the surface of a variety of microorganisms. Activation of the complement pathway via MBL is initiated by specific MASPs. Three MASPs have been identified and all have domain structures similar to those of C1r and C1s with a heavy chain (chain A) and a light chain (chain B). Chain A is composed of CUB1, EGF, CUB2, CCP1 and CCP2 while chain B corresponds to the catalytic domain found in many serine proteases. MASP1 and MASP3 are two alternatively spliced products of a single gene, which contain the same A chains but entirely different B chains. Distinct MASPs found in different MBL oligomers may have different biological activities. For example, MASP3, found together with MASP2, downregulates the C4 and C2 cleaving activity of MASP2. The protease activity of MASP3 is first revealed here using rhMASP3CD, which is inhibited by serine protease inhibitors such as Ecotin and AEBSF.