Investigations to establish a relationship between endotoxin contamination in cotton dust and reduced lung cancer were conducted. C57 BL/6J mice were injected in a lateral tail vein with Lewis Lung carcinoma and exposed to aerosolized endotoxin (Escherichia coli) for 160 minutes/day for five days a week using a Pitt #1 glass aerosolizer. Treated mice had significantly reduced lung cancer when compared to controls. The results support the hypothesis that inhaled endotoxin is an active agent for inhibiting metastatic spread of this tumor to the lung. This is in agreement with previous investigations indicating that endotoxin is the agent responsible for reduced lung cancer in both animal and epidemiological studies on cotton dust exposures. The mechanism of cancer inhibition is attributed to tumor immuno-modulation induced by endotoxins.