Doctors Denton Cooley and Christiaan Barnard signed first day cover ''honoring American blood donors''. Barnard - the first surgeon to perform a human-to-human heart transplant, and Cooley - the first to perform an artificial heart transplant, here sign in fine black felt-tip, without inscription. Cooley dates his signature to 3 February 1995. Postmarked 12 March 1971 with First Day of Issue cancellation. Measures 6.5'' x 3.625''. Near fine condition.