Airborne red oak (Quercus rubra) allergens were collected with volumetric air samplers during 1983–1985 and quantitated immunochemically using red oak pollen proteins adsorbed to plastic microtiter plates and pooled IgE-containing sera from seven oak-sensitive persons. Peak levels of allergen occurred in late April to early May and coincided with peak oak pollen counts from Rotoslide collections. However, additional peaks of oak allergenic activity were measured during July through early October, at times when oak pollen was not airborne. Most of the allergen was in particles smaller than 5 microns even during the pollination period. These peaks coincided with changes in wind from the usual northwest direction to a southernly direction. Among 19 other tree, grass, weed, mold, or insect allergens tested, only birch, and, to a lesser extent, willow, cross-reacted with red oak. We postulate that bits of decaying leaves, male catkins during the summer and early fall and other plant particles become airborne during changes in wind direction and velocity. Because other allergens, such as ragweed,Alternaria, grass and insects, are also abundant in late summer and fall, it will be very difficult to determine the degree to which this oak allergen contributes to late summer hay fever and asthma.