Segments in the Drosophila adult are divided into clonally distinct anterior and posterior compartments. Mutations at the engrailed locus can affect the pattern of cuticular structures in the posterior compartments of segments, but have no obvious effect on anterior structures; for example, bristles that are normally seen only on the anterior wing margin in wild-type flies can be found on the posterior margin of engrailed wings. These and clonal analysis data led to the hypothesis that engrailed causes a transformation of posterior to anterior identity in the wing cells. Despite some striking examples of this transformation, a common engrailed phenotype is the disruption or elimination of posterior pattern elements, without a clear replacement by anterior structures; this, together with indications that localized cell death can mimic some of the observed posterior-to-anterior transformations, has led some investigators to question the original engrailed hypothesis. Recently, monoclonal antibodies displaying region-specific binding patterns on the wing imaginal disk have been described, and one of these antibodies in particular provides a novel probe for the engrailed phenotype in the larval precursors of the adult wing. Here I compare the antibody binding patterns on engrailed and wild-type wing disks. The results strongly support the notion that engrailed mutations cause a posterior-to-anterior transformation in these cells.