An animal model for evaluation of endotoxins was developed in mice. Lethality was measured over a three day period. The sensitivity of the mice to endotoxin was enhanced by intraperitoneal Actinomycin D. Lipopolysaccharide (LPS) content of the lungs of surviving and non-surviving mice was measured by Limulus Amoebocyte Lysate testing. The two organisms (Enterobacter agglomerans and Pseudomonas syringe commonly found in cotton from a wide variety of sources were used as a source of lipopolysaccharide. Two preparations of LPS from Enterobacter agglomerans were very similar. A third preparation was quite different as a cause of lethality in the mouse model. LPS from Pseudomonas syringae was less toxic to mice. The major organism contributing to the LPS in cotton and cotton dusts may be Enterobacter agglomerans.