The pathophysiologic mechanisms of pollen-induced asthma have heretofore remained unclear, because pollen particles have generally been considered too large to penetrate into the lower respiratory tract. Such grains are therefore believed to be unable to induce an immunologic response in the lower respiratory tract. Nevertheless, experimental curves of particle penetration into the respiratory tract indicate that a small percentage of large-sized particles may penetrate into the peripheral areas of the lungs. Under the conditions of our study, a radioisotopic method using labeled pollen grains was not effective in detecting the very small number of grains expected to be found in the lung. We used another simpler and more effective method derived from techniques used by palynologists in the detection of pollen grains in sediments. According to our results, which showed pollen grains normally present in tracheobronchial secretions and in lung parenchyma, we can safely assume that inhaled pollen does penetrate into the distal lower respiratory tract. At present, we do not know the precise number of pollen grains necessary to trigger a response, and we do not know the behavior of these grains with regard to the mucous membranes of the small bronchi and the alveoli. It is important to determine whether these grains have an effect on these areas and whether a direct mechanism is involved in the genesis of pollen-induced asthma.