The present investigation is undertaken to demonstrate a novel cross-reactivity between aeroallergens (moulds fungi imperfecti) and allergens from foods (spinach and mushroom Agaricus bisporus). We have performed a dual study in vivo and in vitro, in a population of atopic patients. Data from in vivo tests performed with spinach and mushroom have been statistically analysed. To the in vitro assays, mushroom and spinach extracts have been obtained, and sera from moulds allergic patients analysed by means of IgE-immunoblott assays. Inhibition experiments have been also performed to study a possible relation between proteins. Statistical analysis of data showed a relation between allergenicity to moulds (Alternaria alternata, Cladosporium herbarum and/or Aspergillus fumigatus), and positive skin prick tests with mushroom and/or spinach. The immunoblotts performed showed that seven moulds allergic patients had a strong recognition of a protein with a molecular weight of about 30 kD present both in spinach and mushroom extracts, and by means of inhibition assays we could determine that these two proteins were related. This study demonstrates the existence of a new allergen responsible for cross reactivity between moulds and two frequently consumed foods, mushroom and spinach. We conclude that a novel cross-reactive allergen between aeroallergens and foods has been identified.