The interferon gamma (IFN-gamma) response region of the human class II major histocompatibility complex gene, DPA, has been localized to a 52-base-pair (bp) DNA fragment in the proximal promotor at -107 to -55 bp after transfection into HeLa cells of a series of 5', 3', and gap deletion mutants linked to a reporter gene, human growth hormone, as well as of synthetic oligonucleotides fused to the heterologous promoter thymidine kinase. The 52-mer sequence contains the X and Y box elements conserved in all class II genes; their presence is indispensable for IFN-gamma inducibility. Furthermore, an additional 5 bp immediately 5' of the X box of the DPA gene are necessary and sufficient for IFN-gamma induction. This region may contain an IFN-gamma response element. A closely related sequence has also been found in the vicinity of the critical deletion sites of three other well-studied class II gene promoters, all of which require a much longer sequence 5' of the X box. A fourth element, the W element, located about 15 bp 5' of the X box in all class II genes, is clearly of little importance in IFN-gamma inducibility of the DPA gene.