The use of daily urinary neopterin evaluation to detect immunological complications has been tested in 96 consecutive cadaveric kidney recipients, three liver recipients, and one pancreas recipient. In 29 of these patients an immunologically uncomplicated posttransplant course was associated with stable or low neopterin levels, or both. In only 5% of daily determinations on these patients were increasing or high neopterin levels seen. On the other hand, major immunological complications, such as acute rejection episodes (38 cases), viral infections (17 cases), or both problems (8 cases), were preceded by increasing or high neopterin levels or both--on the average by one day. Withdrawal of cyclosporine was also found to be followed by increase of urinary neopterin levels. Neopterin evaluation enabled reliable and accurate prediction of immunological complications in 95% of patients with acute rejections and in 100% of patients with viral infections. It thus appears that daily assessment of urinary neopterin levels represents a useful tool for biochemical detection of immunological complications in allograft recipients.