Aging eliminated the tolerance-inducing effect of anti-CD45RB on cardiac allografts. Hearts from C3H mice were transplanted into the abdominal cavity of B6 mice at different ages and treated with anti-CD45RB antibodies (100 µg, intraperitoneally, on days 0, 1, 3, 5, and 7). (A) In 2-month-old mice, all grafts were rejected rapidly without antibody treatment, but 55.6% of grafts survived beyond 150 days after anti-CD45RB therapy. (B) There were five in eight and four in eight allografts with long-term survival in 6- and 9-month-old mice, respectively; however, no allografts in 12-month-old (MST = 47.1 ± 14.1 days) and 14-month-old (MST = 36.0 ± 18.0 days) mice survived beyond 200 days. *P < 0.05, 6- and 9-month-old mice versus 12- and 14-month-old mice.