We have demonstrated the presence of crystalline particles in University of Wisconsin (UW) solution used to perfuse rhesus monkey kidney transplants. These particles were visible in obstructed blood vessels and associated with immediate graft thrombosis and necrosis. This occurred in 25.7% of kidneys perfused with UW solution and transplanted into young, unsensitized recipients. Two molecular species of crystals were defined by mass spectrometry. The particle size ranged from 3 to greater than 100 μm, with a preponderance of particles less than 25 μm in diameter. Such particles are not removed by 40-μm filtration, but can be removed by centrifugation. With extensive use of UW solution for organ storage, the potential for particle-induced damage in small vessels in both experimental and human transplants needs to be carefully scrutinized.