Abstract Settled grain dust was collected from several active grain elevators in the Superior-Duluth areas of the United States. Particle size distribution (47% <5 μm) and endotoxin contamination (429 ng/gm) of the dust were similar to those reported for the airborne parent dust. Human complement was activated in vitro in a dose-response manner which could be quantified. This hemolytic consumption was via the alternative pathway as defined by ethylenediaminetetraacetic acid/ethylene-glycol-bis-(β-amino-ethyl ether) N,N′-tetraacetic acid (EDTA/EGTA) differential serum chelation, factor B conversion, and complement reductions in serum from guinea pigs deficient in C4. It is proposed that continuous low-dose exposure to aerosolized, biologically active rafler dust could contribute to the respiratory insult of grain workers.