S02E22 Blood Feud
Mr. Burns is dying of hypohemia, and needs an immediate transfusion of double-O negative blood. Finding out that Bart has the right blood type, Homer volunteers him, confident that Burns will reward him richly. When all Burns sends is a thank-you card, an enraged Homer writes a hate-mail letter. Marge begs him not to mail it, but Bart does it the next morning.