C-1s, one of the two serine proteases of C-1, the first component of complement, has the ability to mediate heterologous (C-1r-C-1s) as well as homologous (C-1s-C-1s) Ca(2+)-dependent interactions both involving the NH2-terminal alpha region of its A chain. Lactoperoxidase-catalyzed iodination of C-1s in its monomeric form was found to abolish its ability to form Ca(2+)-dependent homodimers, without impairing its ability to mediate C-1r-C-1s heteroassociation. C-1s iodinated in its dimeric form, in contrast, fully retained the ability to self-associate. With a view to identify the tyrosine residues iodinated in each case, C-1s was radioiodinated in its monomeric and dimeric forms, and comparative tryptic mapping was performed on the resulting 125I-labeled A chains. Most of the tyrosine residues either were not iodinated or were equivalently but not in the dimer. Conversely, Tyr-52 and Tyr-147 were iodinated only in the dimer. These results provide further evidence that the structural determinants of C-1s required for Ca2+ binding and Ca(2+)-dependent protein-protein interactions are contributed by both the NH2-terminal motif I (positions 1-110) and the epidermal growth factor like motif II (positions 111-159) of the alpha region. On the basis of available information, tentative models of the C-1s-C-1s and C-1r-C-1s Ca(2+)-dependent interactions are proposed.