In 1966, the first Germinal Centre Conference was held in Bern, Switzerland, as a result of an initiative from C.C. Longdon of Oak Ridge, Tennessee and H. Cottier of Bern. Fifteen years later, in June of 1981, the 7th conference in this series was held in Groningen, the Netherlands. At this "International Conference on Lymphatic Tissues and Germinal Centers in Immune Reactions," instead of focusing on germinal centres only, the intricate relationships between various constituents of the lympho-myeloid complex constituted the theme of the meeting. As Yoffee stated in 1966, "Perhaps we should consider not only the germinal centre itself but the surrounding tissue as well." Over the years, these conferences have brought together a small but dedicated group of scientists, sharing (1) a fundamental interest in the physiology of the lymphoid system proper, and (2) the recognition that, only in the in vivo situation can the final answers be obtained, however ingeniously in vitro experiments may mimic reality. In 1966, the conference lasted three days and included 57 contributions "which were discussed at length." The present conference accommodated 62 free papers and 64 poster presentations as well as some 24 additional abstracts selected from more than 200 submissions. Prominent among the selection criteria was the adage "in vivo veritas." This volume represents the presentations from the 1981 Germinal Centre Conference. At the meeting, the organizers restricted topics to those related to structure and function of the lymphoid system including the bone marrow. Thus, a compact meeting, where topics of interest to all participants were presented at plenary sessions, emerged. This formula of plenary sessions, as in previous meetings, seems to be largely responsible for the continuing success of these conferences. At this point, the organizers wish to acknowledge the very stimulating help of Dr. M.G. Hanna, Jr., of the NCI-Frederick Cancer Research Facility, Frederick, Maryland, USA. He was present at the birth of the first Germinal Centre Conference and since then has been the "auctor intellectualis" of every following meeting. It seemed only proper to end the meeting with a session dedicated to "the still unsolved mystery of the germinal centre (continued)," referring to the title of a paper presented by Dr. J.H. Humphrey, as invited speaker, at the fifth conference in Tiberias, 1975. To mark the occasion of his retirement and to acknowledge his many and outstanding contributions to this and previous Germinal Centre Conferences as well as to the immunological society as a whole, Prof. Humphrey was presented with the "7th Germinal Centre Conference Medal" which was especially designed for the occasion. Finally, some questions were answered and new questions arose at this meeting; all of which is part of the usual scientific process. It is our sincere wish, however, that at future meetings the congeniality among participants, so characteristic of the last and previous meetings, will continue to accompany our progress in science.