This study had two objectives. The first was to measure the response of guinea pigs exposed to either a low (1 mg/m³), medium (6 mg/m³) or high (23 mg/m³) concentration of a zymosan aerosol and the second was to determine how the guinea pigs' response to endotoxin exposure (10.7^. 10³ EU/m³) was altered by pre-exposure with a zymosan aerosol (23 mg/m³). The animals' reaction to each exposure was evaluated by measuring both changes in breathing rate in 10% CO₂ in air and the pulmonary cellular response by broncho-alveolar lavage. Results showed that exposure to zymosan alone had very little effect on the guinea pigs' breathing rate response but did exhibit a dose response relationship with respect to the total number of cells and granulocytes recovered by lavage. In addition, the macrophages appeared to be activated by zymosan exposure as evidenced by their ability to produce reactive species when challenged by zymosan in vitro. Results of the second part of the study showed that when compared with the cellular response of animals exposed to endotoxin alone, there was a significant decrease in both the total number of cells and the number of granulocytes recovered in animals exposed serially to zymosan and endotoxin. The macrophages of serially exposed animals appeared, however, to be more activated than macrophages from animals exposed to endotoxin or zymosan alone.