Liposomes prepared from 6.3∶1.8∶0.9∶ 1.0 molar mixtures of phosphatidylcholine, dicetyl phosphate, cholesterol, and phosphatidylserine, respectively, (PS(+) liposomes) were compared with similarly prepared liposomes without the phosphatidylserine (PS(−) liposomes) for their effect on calcium phosphate precipitate formation in aqueous solutions at pH 7.4 and 22°C. The liposomes, encapsulated with 50 mM phosphate (PI), were suspended in buffered 2.2 mM CaCl2, 0 or 1.5 mM KH2PO4 solutions and made permeable to Ca2+ fluxes with the ionophore, X-537A. External solution Ca2+ losses were found to be small in both PS(+) and PS(−) liposome suspensions when no ionophore was added. Even with 1.5 mM PI in the external solution, these losses did not exceed 0.2 mM. However, inoculating both liposome preparations with X-537A resulted in rapid, appreciable losses in solution Ca2+. Previous studies showed that in PS(−) liposomes, these latter losses were due to calcium phosphate precipitation, with the precipitate confined to the interior of the liposomes when no external PI was present, but extending to outside the liposomes when the suspending medium was rendered metastable. In the present study, Ca2+ losses resulting from intraliposomally confined precipitation were found to be marginally greater in PS(+) liposomes due primarily to a larger volume of entrapped PI available for reaction in these liposomes. However, with the addition of PI to the external solution, the reverse was observed, i.e., considerably less Ca2+ was lost in PS(+) than in PS(−) suspensions, a result of markedly less X-537A-induced precipitate forming outside PS(+) liposomes. The most probable explanation for this latter decrease was a PS-induced adherence of the outer liposome membranes to the surfaces of developing crystals, restricting the avialability of these surfaces as sites for further growth.