During the past 5 years rapid progress has been made in the understanding of biochemical pathways for signal transduction in lymphocyte activation. Gene cloning technology has been instrumental in defining and making available in pure form of a number of growth and differentiation factors, in the characterization of their receptors, and in the delineation of genes for the T cell receptor. This book is divided into 6 sections. Section 1 deals with the molecular structure of the T cell receptor. Section 2 discusses the role of the T cell receptor, membrane ion channels and biochemical pathways of signal transduction in T cell activation. The molecular structures and biological and immunological effects of interleukin 1, interleukin 2 and interleukin 3 are presented in Section 3. This section also details the structure of interleukin 2 receptor and its use as a target for therapy for certain leukemias. Section 4 includes the biochemical events which occur following the delivery of the signal for B cell activation, proliferation, and differentiation by antigen, growth/differentiation factors. The molecular structure of B cell stimulating factors is also discussed. The role of oncogene expression in cellular activation and differentiation is included in Section 5. The cellular and molecular basis of natural killing and the molecular basis of cyc1osporin A-mediated immunosuppression are discussed in detail in Section 6. We hope this book will serve as a reference work on basic mechanisms of lymphocyte activation, proliferation, and differentiation for immunologists and molecular biologists.