The guinea pig animal model has demonstrated concentration dependent respiratory responses to inhalation of cotton dusts and has indicated that dusts differ in their potency for causing adverse respiratory reactions. The current study examined factors responsible for these differences by comparing responses of animals to two purified Gram-negative bacteria and to the dusts in which these microbes were prominent. Results indicated that the animals gave similar responses upon inhalation of cotton dust 1182 and MS84 although the dusts contained different proportions of Enterobacter agglomerans and Pseudomonas syringae. By contrast, responses to the purified Gram-negative bacteria, prepared by acetone-extraction of the cells, displayed some differences. Pseudomonas syringae cells produced more severe responses. Most prominent was severe airflow interruption noted with Ps. syringae but to a much lesser degree upon exposure to Enterobacter agglomerans cells.