Control of seasonal symptoms by means of a preventive and easy to use (only one intradermal injection eight weeks before the pollen peak) immunotherapy, is recommended nowadays. We verified the clinical efficacy of E.P.D. (Enzyme Potentiated Desensibilization) in a double-blind, placebo-controlled study. This particular immunotherapy consists of an intradermal injection mix, made up of allergenic extracts at extremely low doses and an enzyme called beta-glucuronidase. The vaccine is administered once a year, eight weeks before pollen peaks. We studied a group of 40 patients allergic to grass pollen. The results, analysed statistically on the basis of a symptoms score, showed good clinical efficacy and a significant reduction of drug consumption during the high pollen period. Due to the clinical effectiveness, easy administration (only on injection) and excellent tolerance of the immunotherapy, E.P.D. is particularly suited for the prevention of seasonal symptoms in patients allergic to grass pollen.