The compound eye of D. melanogaster is a reiterative pattern of facets, each containing eight photoreceptor cells in a precise arrangement. This pattern is established in the eye imaginal disc during the third larval instar. A wave of morphogenesis sweeps from posterior to anterior across the disc, leaving in its wake organized clusters of photoreceptor cells. We have used monoclonal antibodies to highlight pattern elements that are not readily observable by other techniques. Monoclonal antibodies can be used to identify the molecules associated with particular patterns, providing links between observable structures and the genes. As an example, we present the purification and N-terminal sequence of a glycoprotein antigen specific to photoreceptor cells and their axons.