A study of the microorganism, endotoxin, and aflatoxin content of settled agricultural dusts was conducted. Fourteen samples of settled dust were collected from two factories processing rice and wheat straw (hay) near Shanghai, China. These were analyzed for total bacteria, gram negative bacteria, thermophilic actinomycetes, fungi, endotoxin, and aflatoxins. The ability of aqueous extracts of the dusts to induce production of interleukin-1 (IL1) by human peripheral blood monocytes and to activate the human serum complement system was also investigated. The total viable microorganism concentrations ranged from 2.7 x10(7) to 4.1x10(9) colony forming units per gram (CFU/g). Total microorganism counts were generally higher in the hay than rice dust samples, typically ranging from 2.4x10(-9) to 4.1x10(9) and 2.7x10(7) to 2.9x10(7)CFU/g, respectively. The hay dust samples contained higher concentrations of bacteria, thermophilic actinomycetes, and fungi than the rice dust samples. Yeasts were found in most rice dust samples, but not in the hay dust samples. Bacteria were usually present in the highest concentration in both types of dust, followed by fungi and thermophilic actinomycetes in that order. Relative gram negative counts were higher in rice dust, but the absolute gram negative counts were higher in the hay dust. Aspergillus, Cladosporium, Penicillium, Trichosporon, and Cryptococcus were the most predominant fungal species identified. Aflatoxins were not detected in any samples. Elevated endotoxin concentrations were found in all samples, the concentrations being nearly three times higher in the rice dust than in the hay dust samples. Extracts of both dusts induced IL1 production by monocytes to a similar extent. IL1 production was not significantly correlated with the endotoxin content of the dusts. Extracts of both dusts activated human serum complement, hay dust being more potent. The authors conclude that the concentrations and types of bacterial and fungal species found in the dusts and the presence of endotoxins in the dusts suggest that workers exposed to the dusts may be at risk for farmer's lung or other respiratory diseases.