A significant correlation of kidney transplant outcome with the strength of mixed leukocyte culture stimulation was found in a series of 131 cadaver donor transplants. Graft survival rates in patients whose cultures with donor cells resulted in less than 20,000 cpm were statistically significantly better than those in patients with greater than 20,000 cpm (at 6 months and 1 year P less than 0.001). Cultures of recipient with normal control lymphocytes also correlated with clinical outcome, albeit to a somewhat lesser degree. Bidirectional cultures gave a slightly better correlation than unidirectional cultures. Whether the patient's plasma contained factors that blocked the mixed leukocyte culture reaction or not was not found to have a significant effect on transplant outcome.