Serum and urinary neopterin concentrations were measured in 142 patients suffering from various malignant diseases. Increased serum and urinary neopterin levels were found in 48% and 55% of patients respectively. Neopterin showed sufficient sensitivity in the detection of haematological disorders and hypernephroma, whereas the sensitivity of neopterin was rather poor in patients with other solid tumours. Comparison of serum alpha-fetoprotein (AFP) and human chorionic gonadotropin (HCG) concentrations and serum or urinary neopterin levels in testicular cancer patients showed that determination of neopterin provides no further information in the management of these patients. In testicular cancer patients receiving adjuvant chemotherapy, a significant increase in serum (p less than 0.005) and urinary (p less than 0.0005) neopterin concentrations was measured after chemotherapy, reflecting the release of neopterin from macrophages during the damage by cytotoxic drugs. In the adjuvant testicular tumour patients, known to be tumour free, 1 out of 16 patients showed elevated serum and 3 out of 15 patients showed false positive urinary neopterin levels. False positive serum (24%) and urinary (37%) neopterin values were also obtained in 28 patients with various nonmalignant diseases, but no evidence for an inflammatory process. In 23 patients with inflammatory diseases pathological serum and urinary neopterin levels were measured in 55% and 59% respectively. When a RIA was compared with a HPLC method, higher urinary neopterin values were obtained with the RIA, indicating that non-oxidized pterines are also determined by the RIA method. In conclusion, neopterin might be a helpful biological marker in monitoring patients with malignant haematological disorders and renal cell carcinoma, but provides no additional information in other solid tumours studied.