The immunological activity of airborne dusts to which grain storage workers are exposed was studied. Samples of airborne dusts of barley, corn, durum wheat, oat, rye, and spring wheat were collected in active port grain terminals in the Duluth/Superior region of the United States. Aqueous extracts of these dusts were prepared by mixing each sample (10 percent weight/volume) with sterile pyrogen free water for 24 hours at 4 degrees-C. Extracts were then centrifuged, filtered, lyophilized, redissolved at various concentrations, and dialyzed against saline. Endotoxin and protein concentrations of each extract were determined. Lectin activity was determined by agglutination of sheep erythrocytes. The only extract which agglutinated sheep red cells was that of barley dust. Addition of specific antibody to known lectins demonstrated agglutination by the corn and rye dust extracts as well. The lymphocyte blast transformation assay, using spleen cells from Lewis- rats, was used to determine the mitogenic activity of each extract. Dilutions of each extract or of the known lymphocyte mitogens phytohemagglutinin and concanavalin-A were added to the spleen cell suspensions, which were incubated at 37 degrees-C for 96 hours. Cells were harvested and incorporation of tritiated thymidine was determined by scintillation counting. Each extract caused the spleen cells to proliferate, and the amount of thymidine uptake was a function of the concentration of extract used. There was no apparent association between mitogenic potential of the extracts and either protein or endotoxin concentration. The authors conclude that this mitogenic activity might contribute to lymphocyte mediated reactivity in-vivo after inhalation of airborne grain dusts.