Calcium-phospholipid-phosphate complexes (Ca-PL-P) were isolated from calcified and uncalcified Bacterionema matruchotii and its calcified lipid extracts. Similar complexes were absent from the noncalcifying bacterium Actinomyces naeslundii. The majority of the Ca-PL-P complexes were associated with the proteolipid acidic phospholipid component. Ca-PL-P complexes isolated from B. matruchotii and from calcified proteolipid contained phosphatidylinositol, phosphatidylinositol-4-phosphate, phosphatidylinositol-4,5-diphosphate, and phosphatidylserine. They consisted of approximately 52 mole % Ca, 32 mole % organic P, and 15 mole % Pi. During Ca-PL-P extraction from B. matruchotii or its proteolipid-containing calcified lipid extracts, the proteolipid was dissociated and the apoprotein precipitated as fluff at the aqueous-organic solvent interface, thus explaining the failure to detect protein in Ca-PL-P preparations. When the ability of Ca-PL-P complexes and lipid fractions of B. matruchotii to initiate apatite formation from metastable calcium phosphate solution was compared, the yield of hydroxyapatite decreased as follows: Ca-PL-P greater than proteolipid acidic phospholipids greater than proteolipid greater than crude phospholipid greater than total lipids greater than whole cells.