Bacterial endotoxins act at picomolar to nanomolar concentrations to stimulate a wide variety of cell types including phagocytic and endothelial cells. The major elements identified to date that are crucial for recognition of endotoxin are lipopolysaccharide (LPS)-binding protein, membrane-bound CD14 and, most recently, soluble CD14. Recent results also indicate that membrane-bound CD14 is probably one part of a multi-component LPS receptor. An immediate consequence of engagement of this functional LPS receptor is protein tyrosine phosphorylation and initiation of the multiple intracellular events associated with LPS-induced cell activation.