Toxicity of cyclosporine (CsA), an immunosuppressive drug widely used in transplantation, to the transplanted kidney creates a serious side effect. Therefore, searching for sensitive indicators of nephrotoxic action is well worth the effort. In this work we describe the results of estimation of urine concentration of lysosomal enzymes widely present in the kidney: N-acetyl-beta-D-glucosaminidase (NAG), its isoenzyme NAG-B and beta-glucuronidase (beta-Gr). The studies were conducted in various periods after transplantation of kidneys, on patients under various treatments and receiving different doses of CsA. The results indicate a substantial dependence of the activity of NAG and NAG-B on CsA doses and the period after transplantation. The enzyme proved to be also a sensitive indicator of graft rejection. No such dependence was observed in the case of beta-Gr.