Four allergologically important fungi, viz. Aspergillus fumigatus, Penicillium notatum, Alternaria alternata, and Cladosporium herbarum were cultured in a liquid synthetic medium, with or without addition of 0.1% yeast extract (YE). After 10 and 28 days of cultivation, immunochemical properties of the fungal extracts, characterised by precipitation pattern and IgG- and IgE-binding capacity, were studied. In pure synthetic medium no large differences were observed in number of precipitates, as measured by DID, among the four antigenic fractions of each fungus (metabolic and mycelial antigens of both early and late phase cultures). Addition of YE to the growth medium hardly changed the number of precipitates and generally caused a decrease in IgG-binding capacity of the extracts. In contrast to these observations were the findings with the IgE-binding capacities of the fungal extracts as measured by RAST. Most allergenic fractions demonstrated an increase (sometimes strong) in IgE-binding capacity after YE was added to the growth medium. It is concluded that the time of cultivation influences the immunochemical characteristics of fungal extracts and that mycelial as well as metabolic antigens of both early and late phase cultures should be used in order to obtain a wide spectrum of allergens in extracts used for diagnostic purposes.