Differential survival times of various organ allografts in the rat across the same histocompatiblity barrier were studied by transplanting the kidney, heart, intestine, pancreas, and skin from (BN X Le)F1 hybrid donors to Lewis recipients. Some (one-third) of the kidney grafted rats survived for a prolonged period of time (32-72 days, plus one rat surviving over 9 months), whereas all other organs and skin were promptly rejected between 7 and 21 days. Possible factors responsible for the prolonged kidney survival are discussed; the reason for this was not clear but was not related to the period of operative ischemia or postoperative blood-urea-nitrogen, nor were animals tolerant to donor antigen as evidenced by the popliteal lymph node weight assay and signs of mild rejection on histology of grafted kidneys. A hypothesis of autoenhancing mechanism is presented.