We have generated a monoclonal antibody that recognizes a major component of a specialized extracellular matrix in Drosophila imaginal discs. In mature larvae, antibody binding is observed almost exclusively on imaginal discs. On the basal surface of the thoracic discs, the antigen is localized to particular regions of the epithelium, and ultrastructural studies indicate that the antigen is found in a fibrous network secreted between the cells and the basal lamina. The localized expression indicates that the matrix is not simply related to disc differentiation, as all regions of the columnar disc epithelium are determined to secrete adult cuticle. A correlation of the antigen distribution with known developmental events leads us to propose that the antigen-containing network provides an extensible matrix for the rapid elongation of the disc epithelium during evagination; consistent with this, the antigen is a component of the matrix between the dorsal and ventral surfaces of the evaginated wing pouch. The antigen is very large (greater than 5 X 10(5) Da), can be labeled metabolically with methionine and sulfate, and is digested by chondroitinase ABC; these biochemical characteristics indicate that the antigen is a proteoglycan.