Rabbits given acute serum sickness (ASS) and treated with Cyclosporin A (CyA) developed glomerular capillary thrombosis and cortical infarction. These lesions were associated with severe endothelial injury and platelet-fibrin-leucocyte thrombi. They were not seen in untreated rabbits with ASS, nor in normal rabbits given CyA alone in equivalent doses. A similar renal lesion has been reported in patients receiving CyA following bone marrow transplantation.