The infusion of large numbers of antigen-specific cytotoxic T lymphocytes (CTLs) is a promising therapeutic strategy in the fight against certain cancers and infectious diseases. This process currently involves the time-consuming differentiation or stimulation of natural antigen-presenting dendritic cells from the bone marrow or peripheral blood, respectively, of individual patients. These cells must then be pulsed with antigens to condition them to induce antigen-specific cytotoxic T cells in vitro. In this issue, Latouche and Sadelain (p. 405) describe an alternative method: the use of stable artificial antigen presenting cells (AAPC). They derived xenogeneic murine fibroblasts expressing specific human HLA class-I peptide complexes along with CD80, CD54, and CD58 by retroviral transduction. These cells could be efficiently expanded in vitro and used to stimulate T cells of any patient of a given human leukocyte antigen (HLA) type. These workers demonstrated potent induction and expansion of CTLs in the context of HLA type A2.1, which is present in 40% of the Caucasian population.