Calcium-dependency of the bead-assay. The graph displays the dependency on calcium ions of the bead agglutination caused by SP-A. Serum samples of four test subjects (SP-A concentrations see legends to figs. 1 and 2) were incubated with beads, anti-goat antibody, goat-anti-human antibody in a buffer containing calcium ions first without EDTA (black square) and then with 50 mM EDTA (black triangle). Each experiment was repeated five times. Removal of calcium by chelation with EDTA leads to smaller bead agglutinates.