Sporulating colonies of Aspergillus fumigatus, A. flavus, and A. niger were subjected to immunofluorescence using specific polyclonal antibodies against the aspergillopepsin PEP (EC 3.4.23.18), a secretory aspartic proteinase produced by A. fumigatus. The proteinase antigen was found mainly in developing conidiophores of aspergilli, in submerged mycelia and on the tips of growing aerial mycelia. Mature aerial hyphae and spores showed no immunofluorescence at all. Sporulating conidiophores revealed only weak activity in A. fumigatus and A. flavus. The distinct pattern of expression of the aspartic proteinase antigens suggests a role for such enzymes in the growth of hyphae and the development of conidiophores and thus for the sporulation process in aspergilli.