An investigation was made of the ability of an artificial cotton dust composed of Pseudomonas syringae bound to cellulose powder to reproduce the "Monday effect" characteristic of cotton dust. A group of four outbred guinea pigs was exposed to the dust for five days, given a two day break, then re-exposed to the dust on a "second Monday". The respiratory pattern observed was similar to that seen upon inhalation of cotton dust, i.e., increased respiratory rate, decreased tidal volume, airflow interruptions and increased numbers of apneic periods. Although the responses were small, measurements of respiratory frequency in the presence of 10% CO₂ gave indication of a "Monday" response.