A protein allergen has been isolated from mustard seed extracts by gel filtration. In RAST-inhibition experiments the allergen inhibited more than 60% of the binding of mustard-specific IgE from a pool of seven patients' sera. The allergenic activity was also demonstrated by histamine release from whole blood from two sensitive patients. The allergen had an isoelectric point close to 10.5 and an apparent molecular weight of 14,000. It was stable in alkaline and acid solutions and showed resistance to heating and to degradation by proteolytic enzymes.