Proteins of the PAT family, named after perilipin, adipophilin, and TIP47 (tail-interacting protein of 47 kDa), are associated with lipid droplets and have previously been localized by immunofluorescence microscopy exclusively to the droplet surface. These proteins are considered not to be present in any other subcellular compartment. By applying the high resolution technique of freeze-fracture electron microscopy combined with immunogold labeling, we now demonstrate that in macrophages and adipocytes PAT family proteins are, first, distributed not only in the surface but also throughout the lipid droplet core and, second, are integral components of the plasma membrane. Under normal culture conditions these proteins are dispersed in the cytoplasmic leaflet of the plasma membrane. Stimulation of lipid droplet formation by incubation of the cells with acetylated low density lipoprotein leads to clustering of the PAT family proteins in raised plasma membrane domains. Fractures penetrating beneath the plasma membrane demonstrate that lipid droplets are closely apposed to these domains. A similar distribution pattern of labeling in the form of linear aggregates within the clusters is apparent in the cytoplasmic monolayer of the plasma membrane and the immediately adjacent outer monolayer of the lipid droplet. The aggregation of the PAT family proteins into such assemblies may facilitate carrier-mediated lipid influx from the extracellular environment into the lipid droplet. Lipid droplets appear to acquire their PAT proteins by interaction with plasma membrane domains enriched in these proteins.