During the last 6 1/2 years, serum AFP has been determined by radioimmunoassay in 387 patients with germ cell tumors of the gonads and extragonadal sites. The histological appearances of all these neoplasms were carefully reviewed. Highly elevated levels of serum AFP were noted in patients with tumors containing endodermal sinus (yolk sac) tumor elements irrespective of the location of the neoplasm or presence or absence of metastatic disease. There was good correlation between the presence and quantity of endodermal sinus (yolk sac) tumor elements within the primary tumor or its metastases and elevated levels of serum AFP. All patients with tumors composed of pure seminoma or dysgerminoma, and teratoma, had normal serum AFP levels. Slightly elevated levels of serum AFP up to 60 ng/mg (upper limit of normal 20 ng/ml) were noted in a few patients with testicular tumors composed of pure embryonal carcinoma, whereas patients with tumors composed of or containing endodermal sinus (yolk sac) tumor elements had serum AFP levels that could be measured in 100's or 1000's of ng/ml. Serum AFP was elevated only in patients with active disease. Serum AFP was determined in 81 patients with gonadal tumors of non germ cell origin and was normal in all these patients. Serum AFP is a very good tumor marker in patients with germ cell tumors composed of or containing endodermal sinus (yolk sac) tumor, irrespective of their location. Serial serum SFP determinations can be used for diagnostic purposes, for monitoring the results of treatment, and for early detection of metastases and recurrences. Serial serum AFP determination is a useful procedure in all patients with germ cell neoplasms and is highly recommended.