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.