The ability of cultured kidney epithelial cells (KEC) to secrete neopterin, which is a marker of the activation of immune system was studied. In this study inlerferon gamma (IFN‐γ) was shown to induce neopterin release from KEC in a dose‐ and time‐dependent manner. Many other cytokines and mitogens (IL‐1β, IL‐2, IL‐6, LPS and phorbol ester) were tested for their ability to induce neopterin in KEC but all failed to induce a significant release of neopterin from KEC. By itself TNF‐ á induced a release of small amounts of neopterin but strongly potentiated the effect of IFN‐γ in a synergistic manner to induce neopterin secretion. These data indicate that not only monocytes and macrophages which it is well known secrete neopterin, but KEC are responsible also for the high serum or urine level of neopterin observed in patients with kidney allograft rejection or infections episodes. As the amount of neopterin released by KEC is smaller than that secreted by activated macrophages, the contribution of KEC to the overall production of neopterin during certain di