Although cyclosporine has high specificity for the immune system, immunosuppressive therapy with CsA is often complicated by nephrotoxicity. The main morphologic targets of CsA nephrotoxicity include the tubular epithelial and endothelial cells. These cells were investigated in vitro. CsA caused a dose- and time-dependent inhibition of cell growth, vacuolization and fatty change in adherent cells, detachment, and cell death. Inhibition of 3H-TdR incorporation in cells of both tubular epithelial and endothelial origin occurred between 3 microM and 10 microM. Electron microscopy studies revealed cellular swelling, dilatation of the endoplasmic reticulum, and the presence of lipid droplets and giant mitochondria. The content of the main CsA-binding protein, cyclophilin, in these cell-lines was 5-10 micrograms/mg protein and did not differ in various cell lines, including T cells. Immunohistochemistry using rabbit anticyclophilin antibody revealed diffuse distribution of cyclophilin in the cytosol, nuclear membrane, and nucleolus. Whereas lymphoid cell functions are inhibited at 10-100 nM, CsA had no effect on tubular epithelial and endothelial cells at these concentrations. At concentrations of 3-10 microM, CsA caused growth inhibition and cytotoxicity on cells of lymphoid and nonlymphoid origin. Present evidence shows little, if any, relationship of side-effects to the mode of action of CsA.