Inhalation toxicology studies of endotoxin have primarily focused on either purified LPS or freshly grown, whole bacterial cells. In nature, inhalation of these agents probably is mostly in the form of cellular fragments, senescent cells, and derived cell wall materials. Previous electron microscopic studies of E. agglomerans have shown the existence of small (50 nm) extracellular particles being produced by cell walls which we proposed to be endotoxin production. Through the use of a polyclonal antiserum and the technique of immunogold labelling, we have identified the cellular and extracellular sources of lipid A containing particles. We believe the presence and abundance of these particles in inhaled organic dusts is toxicologically significant.