Guinea pigs were exposed to artificial cotton dusts composed of cellulose powder and purified preparations of Enterobacter agglomerans, Pseuriomonas syringae and corynebacteria to compare the respiratory response to these materials with that previously observed with cotton dust. Following a 6 hour exposure, it was observed that powder containing E. agglomerans and Ps. syringae elicited acute pulmonary reactions qualitatively comparable to that elicited by cotton dust. On an endotoxin basis, the two artificial dusts were equally potent. Dust containing corynebacteria failed to produce a respiratory response. The ability of both artificial dusts, which contained gram negative micro-organisms, to reproduce each of the pulmonary reactions typical of cotton dust exposure suggests a role for gram negative bacteria in the etiology of the acute respiratory response to cotton dust .