The specification and maintenance of the metameric pattern in Drosophila melanogaster is regulated by complicated gene interactions. The differential expression of the homoeotic genes of the Antennapedia complex (ANT-C) and bithorax complex (BX-C), which determine segmental identities, is partly controlled by cross-regulatory interactions of loci within the two clusters and partly by trans-acting factors located outside the two complexes. One of the trans-regulatory genes, Polycomb (Pc), acts as a repressor of the ANT-C and BX-C. Mutations of Polycomb result in a complete depression of the homoeotic genes, leading to abdominal transformations of all body segments. Polycomb is part of a large class of trans-regulatory genes (Pc-group), estimated to comprise up to 40 loci. We have raised antibodies against the Polycomb protein, and, using an improved immunostaining technique, showed that the Polycomb protein binds to 60 discrete sites along the polytene chromosomes of salivary glands. These sites comprise the ANT-C and the BX-C as well as several locations of Pc-group genes. This is the first clear evidence for a direct interaction of Polycomb with homoeotic loci and other Pc-group genes.