Complement regulatory membrane proteins act either on C3/C5 convertase enzymes of the classical and alternative pathways or prevent the formation of membrane attack complexes (MAC). 5I2 is a monoclonal antibody (mAb) directed against a rat erythrocyte membrane inhibitor of the C3 convertase step which seems to be the rat counterpart of mouse Crry/p65. 6D1 is a mAb against rat CD59 which inhibits formation of MAC. Tissue distribution of these membrane inhibitors was visualized by immunohistochemical staining with the appropriate mAb. Rat CD59 (6D1 antigen) and 5I2 antigen were both widely distributed, being predominantly expressed on endothelial cells of the arteries, veins and capillaries as well as on all circulating cells. 6D1 antigen and 5I2 antigen were also detected on immature hepatocytes, systemic endothelial cells, skin fibroblasts, bronchial epithelial cells and bile canaliculi. Both were also expressed in the Schwann sheath of peripheral nerve fibres and ependymal cells. However, glial cells and myelin sheath in the central nervous system were not stained. Anti-CD59 (6D1) staining of epithelial and endothelial cells was observed in the cornea, while 5I2 stained only the epithelial cell layer.