Polysaccharide preparations from Aspergillus fumigatus and Candida albicans when used with some human sera were found to fix complement and precipitate with specific antibodies present in the sera. Precipitins consisted predominantly of IgG with traces of IgM and IgA. Following insolubilization of the fungal polysaccharide preparations with the N-carboxyanhydride of L-leucine, reactivity with serum precipitins was retained and reaction with IgE antibodies was demonstrated using the radioallergosorbent test (RAST). Results showed that for A. fumigatus and C. albicans at least, polysaccharide-specific IgE antibodies probably occur in sera from some hypersensitive individuals. We suggest that chemical methods of the type described which induce polymerization or cross-linking will be useful for detecting IgE antibodies in the RAST. Such an approach could be used with a variety of polysaccharide preparations which cannot be coupled to polysaccharide solid supports by the usual methods.