The rejection of the transplanted allograft is dependent on T cell activation, which requires T cell receptor engagement by antigen and costimulatory signals delivered by T cell surface molecules such as CD28. CTLA4-Ig is a fusion protein that has previously been shown to block the CD28-mediated costimulatory signal and inhibit immune responses in vitro and in vivo. In this report we show that treatment of the C3H/He recipient of a BALB/c vascularized cardiac allograft with a 12-day course of CTLA4-Ig produced indefinite graft survival >> 100 days) in the majority of recipients. In addition, these recipients demonstrated donor-specific transplantation tolerance when tested with donor-specific (BALB/c) and third-party (C57BL/10) skin grafts. These results demonstrate that CTLA4-Ig can induce transplantation tolerance in the adult murine cardiac allograft model.