The cerebrospinal fluid (CSF) and serum of six patients with histologically verified intracranial germ-cell tumors were assayed serially for the presence of alphafetoprotein (AFP) and the beta subunit of human chorionic gonadotropin (HCG). Two patients had embryonal carcinomas, two had choriocarcinomas, and two had dysgerminomas. The marker profile for a given tumor in either CSF or serum correlated with the histological diagnosis; that is, embryonal carcinoma produced AFP and HCG, choriocarcinoma produced HCG, and dysgerminoma produced no markers. The marker levels in serum and CSF declined with therapy and rose usually prior to the development of overt clinical symptoms if the patient's tumor recurred. A CSF-to-serum gradient of the marker levels was present in three of four patients, and the serum levels were often normal when the CSF values were elevated. Ventricular marker levels were lower than the lumbar levels in two of two patients. The assay of these biological markers is a sensitive indicator of the success of therapy, and the presence of a CSF-to-serum gradient suggests that the major portion of the neoplasm rests within the central nervous system. A histological diagnosis can be inferred without the necessity of surgery in appropriate clinical contexts.