SummarySera from 177 atopic patients treated by immunotherapy were screened in order to detect IgD antibodies against a purified fraction ofLollium perenne(fractions C). IgD antibodies were found in 23% of the subjects. Eight of these sera were selected and their IgD isolated by means of immunoabsorption. Thein vitrobiological activity of the IgD antibodies against fraction C was tested by three methods: basophil degranulation, histamine release and RAST inhibition test. In all these assays, the data obtained indicated a possible blocking effect of the IgD antibodies upon the IgEin vitromediated reactions.