Serum alpha-foetoprotein (AFP) and serum carcinoembryonic antigen (CEA) levels were measured, serially whenever possible, in 70 patients attending the Institute of Radiotherapy, Rotterdam, on account of testicular (65) or ovarian (4) germ cell tumours or, in one case, an endodermal sinus (yolk sac) tumour in the mediastinum. In 15 patients the disease was active; in the others it was in remission. Patients with active disease had raised serum AFP levels which correlated well with disease activity; no patient without evidence of active disease had raised serum AFP levels. None of the patients with active disease was found to have raised serum CEA levels. There was no correlation between serum AFP and CEA levels in patients with germ cell neoplasms, but good correlation between serum AFP levels and disease activity. Serum CEA levels did not correlate with disease activity, and serial determinations would therefore not be useful in monitoring progress in this group of diseases.