The human lectin complement pathway activation molecules comprise MBL, ficolin-1, -2 and -3, in complex with associated serine proteases MASP-1, -2 and -3, and the non-enzymatic sMAP. Recently, a novel plasma protein named MBL/ficolin associated protein-1 (MAP-1) was identified in humans. This protein is the result of a differential splicing of the MASP1 gene and includes the major part of the heavy chain, but lacks the serine protease domain. We investigated the direct interactions of MAP-1 and MASP-3 with ficolin-3 and MBL using surface plasmon resonance and found affinities around 5 nM and 2.5 nM, respectively. We studied structural aspects of MAP-1 and could show by multi-angle laser light scattering that MAP-1 forms a calcium-dependent homo-dimer in solution. We were able to determine the crystal structure of MAP-1, which also contains a head-to-tail dimer approximately 146 Angstrom long. This structure of MAP-1 also enables modeling and assembly of the MASP-1 molecule in its entirety. Finally we found that MAP-1 competes with all three MASPs for ligand binding and is able to mediate a strong dose dependent inhibitory effect on the lectin pathway activation, as measured by levels of C3 and C9.