On the basis of results obtained with other malignancies, the usefulness of serum ferritin as a tumor marker in the diagnosis of testicular tumors was studied. In 68 patients with malignant testicular tumors and 22 patients with benign testicular processes, serial determinations of beta-human chorionic gonadotropin (beta-HCG), alpha-fetoprotein (AFP) and serum ferritin were performed. The serum ferritin values showed no relationship to tumor stage, and no difference in nonseminomatous and seminomatous tumors. In particular, no diagnostic improvement was achieved in those cases in which the conventional markers AFP and beta-HCG were unsuccessful (falsely negative results). Furthermore, serum ferritin values were influenced by hepatic factors and anemia, so that serum ferritin - on the basis of the present material-could not be considered a useful tumor marker in testicular malignancies. Further studies on the determination of carcinofetal isoferritin and other new tumor markers are warranted.