Scientists may feel that there are too many meetings these days, and we tend to agree on this until it comes to our own field of interest. In our own areas we would like to hear about other people's achievements and learn whatever may be helpful in the search for answers to our own pet questions. Exchange of ideas, discussions, and critical evaluations are almost as essential to progress as the actual laboratory work. These were the major motives which initiated our own efforts, with the support of Dr. Earle H. Spaulding, the Chairman of the Department of Microbiology and Immunology at Temple University School of Medicine, to bring to gether for the first time specialists from various disciplines who are attempt ing to achieve the same thing, i. e. , the clarification of the chemical and immunological nature of different cellular antigens. Instead of publishing the proceedings of the conference, it was decided that we would attempt to review achievements in the different subjects as well as report the latest developments from our own laboratories. Thus we hope to give scientists involved in this explosively growing field not only an up-to-date report, but also a useful source of relevant references. Despite these efforts and aspirations, we realize that this book is not a complete survey of all known cellular antigens. We tried to present major representations of the most important cell types.