Biliary neopterin concentrations were measured daily in nine liver-transplant recipients during the early post-transplant period (on average, 25 days). Concentrations increased strongly during rejection episodes and decreased quickly after successful antirejection therapy. Contrary to the changes of urinary neopterin, cytomegalovirus infection and hepatitis were not associated with an increase in biliary neopterin concentrations. Therefore, measurement of neopterin in bile fluid and in urine may aid in distinguishing rejection from infectious complications in liver-allograft recipients.