To elucidate the IgE binding site of mugwort (Artemisia vulgaris r.) pollen, pollen grains were frozen and fixed using a cryocut. They were incubated with antibodies according to the following sequence: Sera pool of individuals who showed mugwort-RAST class 3 or 4, biotin-labeled goat anti-human IgE antibody, streptavidin-peroxidase and diaminobenzidine. Then, they were observed under electron microscopy. The control section was incubated with the sera pool from individuals who showed a negative result on a skin prick test to mugwort pollen. Antigenic activity (electrondense line) was noted on the surface of the exine. There was no activity in cytoplasm or the intine layer. The control section was completely free of activity. It was suggested that the IgE binding site of mugwort pollen was present on the surface of the exine.