The antibody produced by the hybrid cell line DK. 1A4 recognizes an antigen present initially on all the epithelial cells of the D. melanogaster wing imaginal disc. This antigen becomes progressively restricted to cells in the dorsal region of the disc during the final larval instar. The presence of the antigen does not correlate with the specific adult structures to which the cells will eventually contribute, but rather with the position of the cells in the disc. In late discs, the line bounding the region in which the antigen persists corresponds to the boundary between the dorsal and ventral compartments are revealed by a clonal analysis of the undifferentiated disc. Together, these data suggest that the antigen's disappearance may be specific to the cells of the ventral compartment of the wing disc.