A collaborative study was initiated between Basel and Irvine Laboratories named above in an attempt to characterize a unique and lethal gastrointestinal toxicity in rabbits associated with cyclosporine administration. Data from both laboratories were combined and analyzed. The rate of weight loss in CsA treated rabbits was found to be a significant linear function of the dose. In addition, animal survival decreased and showed a dose-dependent linear relationship to CsA use. Grossly, all of the animals presented with full stomachs, incompletely digested, dry, hard, rabbit chow. Histopathology could not provide any insight into the mechanisms of this gross finding and remain unclear. The complete similarity of clinical and histopathological results in distant independent laboratories confirms the specificity of this CsA associated toxicity in the rabbit.