Serum SP-1 levels were measured serially in 94 patients with non-seminomatous germ cell tumors to evaluate its clinical significance as a tumor marker. In 12 out of 80 patients (15%) with active tumors serum SP-1 was found to be elevated, whereas serum HCG and AFP in the same sample were raised in 53 and 45% respectively. Elevation of serum SP-1 levels was always associated with raised HCG levels, and with AFP in 7 patients. During chemotherapy, serum SP-1 and HCG disappeared when a complete remission was obtained. In contrast to HCG, serum SP-1 failed to detect tumor progression in two patients. Serum HCG and AFP are superior as tumor markers to serum SP-1.