Abstract Activation of the lectin pathway of complement begins with the activation of mannan-binding lectin (MBL)-associated serine proteases, MASP-1 and MASP-2, which are bound to the recognition molecules, MBL and ficolins. MASPs are Ca2+-dependent dimers. Dimerization and Ca2+-dependent association with the recognition molecules occurs via the first 3 domains, the CUB1-EGF-CUB2 region. The CUB1-EGF-CUB2 (D1-3) regions of MASP-1 and MASP-2, and also their tagged versions, were expressed in E. coli, refolded and purified. The first three domains of MASP-1 are identical with the respective regions of MASP-3 and MAp44, which are also associated with MBL and ficolins. The functionality of the fragments was checked by inhibition of C3 deposition from human serum. Time-course of the dissociation and re-association was examined by size exclusion chromatography. Both refolded proteins are tight Ca2+-dependent dimers, as expected. In buffer containing EDTA MASP-1_D1-3 dissociated to monomers, however it took about 1h to reach an equilibrium. Upon re-calcification dimers were re-formed, but this process was even slower; only after overnight incubation was the dimerization completed. MASP-2_D1-3 showed a somewhat different behavior: dissociation by EDTA was even slower, less complete, and higher MW aggregates also appeared. Heterodimer formation was detected by native PAGE. As modeled by the D1-3 fragments, MASP-1 and MASP-2 can readily form heterodimers after dissociation and re-association, however, in the presence of Ca2+ exchange of subunits is slow between the homodimers. MASP-1:MASP-3 heterodimer formation was modeled by the tagged and untagged D1-3 fragments, and data indicate that subunits of these proteins are readily exchanged even in the presence of Ca2+. The existence of heterodimers influences the current view on the composition of lectin pathway complexes and their activation.