The ability to establish long-term B lymphocyte cultures from patients carrying particular genetic characteristics or with the ability to secrete specific antibodies (1) is an extremely valuable technique. However, there are several basic principles to follow in the approach to this technology. It is the purpose of this chapter to provide all the information necessary to run an efficient and safe Epstein-Barr Virus (EBV) transformation system. More detailed information regarding the use of this technique for the preparation of human monoclonal antibodies is given elsewhere (2).