The Fifth Ir Gene Workshop was held at the Chase-Park Plaza Hotel, St. Louis, MO, August 28-31, 1982; 240 scientists participated in the Workshop. The man uscripts compiled in this book describe the state of the art concerning Ir genes. Although the notion of Ir Genes: Past, Present, and Future has not been ad dressed specifically by each author, the reader is certain to get this flavor from the contributions. In this Preface, we have tried to summarize some of the salient ob servations and discussions from the Workshop. The mUltiple genes of the I region have been defined traditionally by serolog ical analysis of intra-H-2 recombinant mice and the pattern of immune responses to certain antigens developed by these recombinant mice. The application of sev eral new techniques, such as gene cloning and DNA sequencing, production of T and B cell hybridomas, and development of cloned T cell lines has changed this tradition and introduced a new phase into the analysis of the I region, Ia antigens, and Ir genes.