Murine resident peritoneal macrophages were maintained in cell culture in a medium containing 10% lipoprotein-deficient foetal calf serum to which various artificial lipid-containing particles were added. These had a core of oxidizable lipid, generally cholesteryl linoleate, and were stabilized in aqueous suspension by one of a variety of poly-L-amino acids, proteins or polysaccharides. Most particles, except those containing poly-L-lysine or poly-L-arginine (both strongly basic), were readily taken up by the macrophages to form typical ceroid inclusions, the morphological form of which was determined by the nature of the core lipid. The hydrophilic stabilizing component seemed largely irrelevant in this respect. The role of the latter appears largely to be to allow the cellular uptake of lipid, although it may also participate in ceroid formation.