"Infectious tolerance" or inducing immunologic tolerance of infection in allografts is still poorly understood. We investigated whether transfusing blood from LEW.1A rats tolerant of LEW.1W hearts could transmit tolerance to naive LEW.1A rats. In 4 of 6 cases, transfusing blood from tolerant animals was followed by immunologic tolerance of heart transplants from LEW.1W donor rats in LEW.1A recipient animals, whereas transplanting heart grafts that were tolerated in previous transplantations across MHC barriers did not transfer tolerance in major histocompatibility complex (MHC)-incompatible animals. We conclude that in rat heart transplantation, the transfer of immunologic tolerance can be enhanced by transfusing blood from tolerant animals to naive animals before transplantation across MHC barriers.