Cell sap (CS) of the pathogenic fungus Aspergillus fumigatus strain Ag-507 was fractionated by Sephadex G-200 column chromatography. A protein fraction designated CS3 was partially characterized by two dimensional electrophoresis (2-DE) and analytical ultracentrifugation. CS3 consisted mainly of low molecular weight components (14 K-43 K) of the whole CS, and produced one peak in analytical ultracentrifugation with an Sapp of 4.25. CS3 was demonstrated to be different from a previously characterized CS fraction designated as CS2, by 2-DE, and by CS2 and CS3 specific antisera. CS3 gave precipitin reactions with three aspergilloma patient sera and 100% of sera from allergic bronchopulmonary aspergillosis (ABPA) patients. Significantly, three ABPA patient sera reacted with CS3 and not CS2. The CS of A. fumigatus strains Ag-515 and Ag-534, were also examined for the presence of CS3 components as were CS preparations of five additional Aspergillus species; A. flavus, A. fischeri, A. terreus, Neosartorya (Aspergillus) fennelliae, and A. niger, and three fungal taxa; Penicillium notatum, Candida albicans, and Saccharomyces cerevisiae. 2-DE, immunoelectrophoresis and double diffusion (DD) analyses of the CS preparations provided complementary information. The immunochemical similarity of CS2 and CS3 components of different aspergilli appears to reflect the taxonomic relatedness of the aspergilli. Additionally, aspergilli exhibiting CS2 and CS3 components most similar to A. fumigatus strain Ag-507 are more frequently isolated from aspergillosis patients. There may be an association of these components with incidence of involvement of the organisms in aspergillosis. DD analysis of the cross-reactivity of CS of all taxa with ABPA and aspergilloma patient sera supported the 2-DE and absorption data.