Until about 10 years ago the exact mechanisms controlling cellular responses to the endotoxin - or lipopolysaccharide (LPS) - of Gram-negative bacteria were unknown. Now a considerable body of evidence supports a model where LPS or LPS-containing particles (including intact bacteria) form complexes with a serum protein known as LPS-binding protein; the LPS in this complex is subsequently transferred to another protein which binds LPS, CD14. The latter is found on the plasma membrane of most cell types of the myeloid lineage as well as in the serum in its soluble form; LPS binding to these two forms of CD14 results in the activation of cell types of myeloid and nonmyeloid lineages, respectively.