A formidable array of practical problems must be solved at laboratory level before tolerance induction in organ transplantation can be considered a realistic clinical possibility. I believe it is too early for even tentative human experimentation in this field. However, the work of the last decade has shown unequivocally that even adult animals can readily be rendered tolerant of evenexceedingly powerfulantigens. The principle of tolerance has such great specificity and such compelling elegance when compared with present-day aids to organ transplantation that intensive effort must go into harnessing it to clinical use. It will certainly enter the clinical homograft scene within a decade, and xenografts are inconceivable in its absence.