In 16 patients with a renal allograft the activity concentrations of aspartate aminotransferase and alanine aminotransferase and the percentage stimulation of both enzymes were investigated. After the transplantation the patients received prednisone and cyclosporine A as immunosuppressive therapy, while exactly 3 months after the date of transplantation prednisone and azathioprine were given as immunosuppressives. In the first period, the percentage increase of the activity concentration of aspartate aminotransferase and alanine aminotransferase upon supplementation of pyridoxal-5'-phosphate in vitro were similar to that of healthy individuals. In the second period, however, the percentage increase of the activity concentration of alanine aminotransferase was much higher than that of aspartate aminotransferase. Cyclosporine A given during a period of about 400 days did not influence the percentage increase of both enzymes. It is concluded that the high stimulation of alanine aminotransferase in the second period depends on the presence of azathioprine or its metabolites in serum.