This chapter reviews that in order to elucidate the molecular mechanisms involved in the induction of cytolytic T lymphocytes (CTLs) in the immune response, it is helpful to be able to separate the antigens expressed on the cell surface, and to test their individual abilities to act as stimulators of and targets for the action of CTLs. It discusses that human histocompatibility antigens at various stages of purification have been used to generate a CTL response. Isolated lymphoblastoid cell membranes and the dialyzed, detergent-soluble fraction from isolated membranes are both effective in eliciting CTLs. The chapter also explores that recently; HLA-DR antigens (DR) have been purified by immune affinity chromatography and used to generate CTLs. The use of purified DR to generate DR-specific CTLs reduces the chances that minor contaminants of the preparation, such as class I antigens, lentil lectin, or cytoskeletal proteins will affect the results. Purified antigen also facilitates analysis of both the requirements for generating CTLs and their specificities.