Mycoprotein ('Quorn') is a food produced for human consumption from Fusarium graminearum. Crossreactivity studies showed that mycoprotein shared multiple common allergenic determinants with Aspergillus fumigatus and Cladosporium herbarum and some with Alternaria alternata. There is, therefore, a potential for mould allergic patients to react adversely to inhaled or ingested mycoprotein. Mycoprotein RAST screening of mycoprotein production workers was made during a 2 year period. Two of the production workers had specific RAST binding > or = 2% but none reported symptoms. Two of 10 patients referred to hospital following vomiting and diarrhoea after ingestion of mycoprotein had a mycoprotein skin-prick test weal > or = 2 mm but none had a significantly raised RAST. These largely negative results are important and reassuring because consumption of the product in the U.K. is now widespread and increasing.