Aspergillus fumigatus conidia exhibited the ability to bind purified human fibronectin, whereas mycelial forms did not bind the ligand, as detected by an indirect immunofluorescence assay with an antifibronectin polyclonal antibody after incubation of the cells with fibronectin. Flow cytometry confirmed that binding of the ligand to conidia was dose dependent and saturable. Pretreatment of the cells with trypsin markedly reduced binding, which suggested a protein nature for the binding sites present at the surface of conidia. Intact conidia were also able to adhere to fibronectin or antifibronectin antibodies, a significant reduction (from 88 to 92%) in the binding of conidia was noticed, thus suggesting that adhesion to the immobilized ligand was specific. Analysis by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and Western immunoblotting with fibronectin and antifibronectin antibody of whole conidial homogenates and 2-mercaptoethanol extracts from isolated conidial cell walls allowed identification, among the complex array of protein and glycoprotein species present in both cell-free preparations, of two polypeptides with apparent molecular masses of 23 and 30 kDa which specifically interact with fibronectin.