Complement activation by commercially prepared allergen extracts of six cereal grains were studied. Standard allergenic preparations of whole ground barley, maize, oat, rye, sunflower seed and wheat were tested. All extract preparations were assayed with spectrophotometric modifications of the Limulus amebocyte lysate gel test. Pooled normal human serum was negative for counterimmunoelectrophoresis for specific precipitating antibodies to the extracts and negative for specific immunoglobin-E to barley, oat, rye, and wheat. All six extract preparations contained gram negative bacterial endotoxin like activity ranging from 2.2 to 38.7 nanograms per milliliter. Protein nitrogen concentrations ranged from 0.6 to 31.2 milligrams nitrogen per milliliter. Alternative pathway activation was observed during complement component C3 conversion in the presence of ethylene-glycol-bis-(beta-amino-ethyl- ether)N,N'-tetracetic-acid, while classical complement components C1, C4 and C2 were decreased in serum free of antibody. Small amounts of allergen extract initiated formation of complement dependent factors which were chemotactic for human polymorphonuclear leucocytes. The authors conclude that there is a potential for complement activation invivo by ingestion, inhalation or injection of cereal grains.