This study evaluated the mixed leukocyte culture (MLC) technique to quantitate differences in kidney allograft survival between unrelated animals. Each of five beagles received two heterotopic kidney allografts, one from a mongrel dog showing high MLC stimulation and one from a second mongrel dog showing a low MLC stimulation. Low or high stimulators were defined by the amount of radioactive thymidine incorporated by lymphocytes when the recipient was tested against a number of potential donors in one way MLC TESTS. Low and high stimulators usually differed by a factor of up to ten in the stimulation index. Recipients were given a standard treatment of asathioprine and prednisone. No special treatment was given for rejection episodes. Creatine clearances and periodic biopsies were used to determine rejection. Creatine clearances and biopsies showed the high stimulating kidneys to be rejected completely by the end of 2 weeks in all animals. All of the low stimulating kidneys maintained normal function for 3 weeks and then developed progressive deterioration and rejection over the next 3 to 6 weeks. These studies suggest that quantitation of the MLC may be predictive of kidney allograft survival in unrelated dogs.