The hydrolysis of birch and timothy pollen allergen preparations by duodenal juice from an adult volunteer was studied in vitro with the aid of rocketimmunoelectrophoresis, radiorocketimmunoelectrophoresis and RAST- inhibition. The results achieved with all methods were similar. The duodenal juice had the capacity to hydrolyse both the grass and tree pollen proteins, and after 30 min preincubation only 10 % of the original birch and timothy allergenic activity remained. Heat treatment of the duodenal juice completely destroyed the proteolytic activity (i.e. no hydrolysis occurred) while protease inhibitors only marginally prevented the hydrolysis.