The subserosal stroma of hernia sacs consistently contains birefringent particulate material, in amounts greater than those observed in other intra-abdominal organs. The major component of this material was shown in the present study to be talc; thus, it cannot be of endogenous origin. Cellular response to this foreign material is remarkably slight. Possible sources of the material and mechanisms of access to the hernia sac were examined in a search of the available literature. It is proposed that the probable source is ingestion with food or, more likely, medications and that the particles reach the peritoneal cavity by migration through the intact intestinal wall. They probably reach the hernia by sedimentation in peritoneal fluid and subsequently migrate into the subserosa. The virtual absence of response to the particles is attributed to their composition (silicate) and their relatively small size (up to about 10 microns) compared with the particles in talc granulomas (up to at least 50 microns).