Bulk samples of silage, burnt hay, and chopped hay were collected from farms in upstate New York following incidents of respiratory illness. Inhalation exposure of guinea pigs to these dusts resulted in elevated breathing rates, inflammation, and activation of alveolar macrophages (Castranova et al., 1992). The present objective was to determine if direct activation of alveolar macrophages by agricultural dusts might play a role in these pulmonary responses. Bulk samples were ground into a powder and alveolar macrophages exposed in vitro to 0.1 - 1.Omg/ml of this material. Silage stimulated oxygen consumption ( 18%) and chemiluminescence ( 91%) of alveolar macrophages. In contrast, burnt hay and chopped hay did not activate these cells. The aqueous extract of silage was more potent than silage powder. Boiling deactivated this extract. Silage, burnt hay, and chopped hay inhibited FMLP-induced respiratory burst activity by 69%, 58%, and 66%, respectively. This inhibition was not due to decreased cell viability. These results indicate that it is unlikely that direct activation of alveolar macrophages plays an important role in the pulmonary response to inhalation of these agricultural dusts.