Airborne grass pollen in a grassland was measured using a Burkard air sampler and immunoblotting. The pollen antigens collected on the tape of the air sampler were transferred onto a nitrocellulose membrane, and visualized as purple spots by immunoenzymatic staining with anti-Lol pI antibody. These spots were quantified automatically using an image processing system. The airborne grass pollen antigens in the grassland where four species of grass were growing were prevalent mainly from evening to early morning during the first half of the pollen season, but in the daytime during the latter half of the season. The antigen dispersion time was confined to a few hours of the day. The flowering time of several grass species which were hydroponically cultured in the laboratory were also studied. Each species had a different flowering time. We believe that the time when an antigen is present during the day is related to the different flowering times of the various grass species growing in the grassland.