Outdoor air-bearing natural ragweed pollen burdens were drawn through 5 microns membrane filters during 24 hr periods; aerosols penetrating these barriers were collected on individual, in-line, 0.8 microns filters. The 0.8 microns filters were free of intact pollen grains and grain fragments when scanned in toto by transmitted light. Aqueous extracts of 0.8 microns filters were found to elicit wheal-and-flare skin test reactions in a ragweed pollen-sensitive subject but not in nonatopic controls. The same extracts produced significant inhibition of an IgG-ELISA system by use of ragweed pollen protein conjugated to polystyrene plate wells and pooled serum of patients on ragweed immunotherapy. No inhibition resulted from preincubation of serum with an unrelated allergen (hickory pollen extract). These data indicate the presence of airborne ragweed pollen allergen in aerosol fractions below 5 microns during seasonal periods of ragweed pollination, confirming the report of Busse et al. Potential sources of such aerosols are prominent in nature, and their occurrence may help explain existing dose-response disparities in pollinosis.