Nanometer-sized gold crystallites were precipitated within multilamellar egg yolk phosphatidylcholine (PC) vesicles by incubation of lipid dispersions with aqueous (\mathrm{HAuCl}_{4}). In situ reduction to (\mathrm{Au}(0)) occurred rapidly in the presence of the lipid molecules. The vesicles and crystallites were characterized by transmission electron microscopy of air-dried stained and unstained samples, and of fixed and sectioned material. Intravesicular crystallites were spherical in morphology, had smaller diamelers (mean (=4.1 \mathrm{~mm}) ), and were less aggregated as compared wilh Au crystals formed in the extravesicular phase (mean diameter (=25.1 \mathrm{~nm}) ). The influence of changes in the membrane composition on crystal nucleation and growth was investigated by the addition of secondary lipids to the PC vesicles. Incorporation of stearylamine resulted in a marked increase in the number of intravesicular (\Lambda u(0)) crystallites, particularly within the interlamellar regions, suggesting that the positively charged amine headgroups provide an energetically favorable site for (\mathrm{Au}() III) reduction and (\mathrm{Au}(0)) nucleation. In contrast, only limited intravesicular crystallization occurred in PC vesicles containing negatively charged lipids such as phosphatidylserine, phosphatidylglycerol, or phosphatidic acid. 1993 Academic Press, Inc.