The correlation between serum alpha-foetoprotein (AFP) and the clinical pathological finding of 24 germ cell tumours arising from the testes (14 cases), the ovaries (3 cases), the mediastinum (3 cases), the retroperitoneal region (2 cases), and the sacrococcygeal region (2 cases) are presented. Irrespective of marked differences in age and sex of the patients, primary site of the tumours and clinical outcome, the 24 cases constituted a homogeneous group in fundamental histological patterns and in AFP synthesis. In all cases of endodermal sinus tumour or teratocarcinomas with a distinct vitelline component an increased serum AFP concentrations was found in the pre-operative serum samples. AFP was also demonstrated in the tumour tissue by quantitative determination of AFP in tumour homogenate (5 cases) and, by immunofluorescence technique, positive staining of the cells lining the endodermal sinuses and of the hyaline globules was found (3 cases). In 12 germ cell tumours without vitelline components in the tumour tissue sections, a normal AFP concentration below 20 mug/1 was found in preoperative serum samples.