The effect of Mg on hydroxyapatite proliferation induced by phosphatidyl serine, phosphatidyl inositol, and calcium-acidic phospholipid-phosphate complexes has been studied in metastable calcium phosphate solutions of constant ionic strength and variable Mg/Ca ratio. Mg inhibits formation of the Ca-acidic phospholipid phosphate complexes, probably by competing with Ca for sites on the phospholipid molecules. Once the complexed acidic phospholipids were present, Mg has no effect on the proliferation of hydroxyapatite. This is shown by the invariant first-order rate constant for the disappearance of CA during hydroxyapatite proliferation (kCa = 0.0037 h-1) in solutions with Mg/Ca weight ratios ranging 0/1 to 10/1. These studies suggest that the presence of Mg does affect in vivo calcification and that the initiation of calcification by means of a Ca-PL-PO4 complex may be dependent on the Mg/Ca ratio in the calcifying tissue.