In order to identify the site(s) on major histocompatibility molecules recognized by cytolytic T lymphocytes (CTLs), the recognition of H-2 antigens expressed when cloned genes are introduced into mouse L cells by DNA-mediated gene transfer has come under investigation. Recently, recombinant H-2 genes have been constructed in vitro from restriction endonuclease fragments of cloned H-2Dd and H-2Ld genes which exchange the N and C1 external domains (exon shuffling). These hybrid H-2 genes direct the synthesis of hybrid H-2 antigens when introduced into L cells by DNA-mediated gene transfer. These transformed L cells have been used as target cells to achieve a more precise localization of the sites recognized by allospecific and virus-specific CTLs. CTL systems were chosen that allow one to probe allospecific Ld or Dd recognition or virus-restricted Ld or Dd recognition. Using this approach we were able to map essential CTL recognition sites to the N and C1 domains of class 1 molecules.