Information about histocompatibility antigens is expanding so rapidly that it is difficult to remain abreast of aB advances. In these volumes, we have made an effort to bring together the most current work on topics that have generated most of the re cent advances and discussions. We have asked each author to present and interpret his most current work, and we have judiciously refrained from imposing our own prejudices and viewpoints. Although there is obvious overlap in some individual topics, we have encouraged this to provide the reader with as many different and some times opposing viewpoints as possible. This approach will, we liope, give a broad overview of current ideas in the field. We wish to thank all contributors for their timely and exciting manu scripts, and we sincerely hope that the reader will benefit from these volumes. R. A. Reisfeld S. Ferrone La Jolla ix Contents I. Serology and Genetics Chapter 1 Studies of HLA-DR Antigens by Complement Fixation Jacques M. Colombani, Laurent Degos, Virginia Lepage, Helene Dastot, Muriel Reboul, and Pierre Lethielleux 1. Introduction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 2. Material and Methods. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 3. Results and Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 3. 1. Screening for Anti-HLA-DR Complement-Fixing Sera . . . . . . . . 4 3. 2. Expression of HLA-A, -B and -DR Antigens on Peripheral Blood Lymphocytes during Phytohemagglutinin Stimulation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 3.3. Expression of HLA-A, -B and -DR Antigens on Peripheral Blood Lymphocytes during Concanavalin A Stimulation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 3. 4.