Background. C57BL/6 mice transfected with the Ld gene coupled to the α-myosin heavy chain promoter result in transgenic mice with Ld antigen expressed only on cardiac tissue. These transgenic animals allow the examination of immune reactivity against cardiac Ld by "self" or by adoptively transferred Ld specific 2C cells, and the response of nontransgenic C57BL/6 mice to the transplanted Ld+ heart. Methods. Naïve cardiac Ld+ transgenic mice were examined for evidence of Ld "autoimmunity." Forty million fresh 2C cells or 2C cells sensitized in vitro for 7 days against Balb/c (Ld+) + interleukin-2 were also given intravenously to Ld+ transgenic mice. At 5 and 12 days after injection, heart-infiltrating lymphocytes were analyzed by fluorescence-activated cell sorter. The Ld+ transgenic hearts were also transplanted to syngeneic Ld- nontransgenic C57BL/6 to evaluate the heart's immunogenicity. Results. Naïve Ld+ transgenic mice did not exhibit any evidence of lymphocytic infiltration on histologic examination. Adoptive transfer of either fresh or in vitro sensitized 2C cells was also unable to reject the native Ld+ heart in transgenic mice (100% of the mice survived long term [more than 60 days]). Sensitization of the Ld+ transgenic mice with a Balb/c skin graft and interleukin-2 pump infusion (7 days) beginning 1 day before 2C cell injection also did not promote rejection of the native Ld+ heart. However, fluorescence-activated cell sorter analysis did reveal that a significantly greater number of in vitro sensitized 2C cells homed to the Ld+, but not Ld-, heart after both 5 and 12 days (P < .01, P < .001). In contrast, C57BL/6 mice rejected the Ld+ (C57BL/6 background) transgenic heart in a mean survival time of 17 ± 9.7 days (P < .01), whereas a syngeneic C57BL/6 heart transplant was accepted indefinitely. Lymphocytic infiltration consistent with rejection was present in all animals receiving an Ld+ transgenic heart transplant, whereas no infiltrate was present in those receiving a syngeneic C57BL/6 heart transplant. Conclusions. Although the class I Ld transgene is not recognized in its native host, its immunogenicity is shown by the homing of anti-Ld 2C cells to the heart in situ and rejection of Ld+ heart grafts when transplanted into syngeneic C57BL/6 mice.