In this volume, in vitro models for the study of lymphoid cell functions and methods for the study of lymphoid cell receptors are presented. Lymphocyte in vitro transformation is discussed in the first section that describes methods for the in vitro stimulation of lymphocytes. Some special media for the study of lymphocyte transformation are also discussed. In vitro methods have been very useful for the understanding of the differentiation of lymphoid cells. The more commonly used of these methods are presented in the second section. The third section deals with the receptors of lymphoid cells.