In this paper, the relationships between the hormone levels in urine and plasma before semicastration, before radiotherapy, and after radiotherapy were studied in five patients with malignant teratoma of intermediate type B and three patients with malignant trophoblastic teratoma. Estrogen levels were significantly elevated preoperatively in all cases of malignant teratoma of intermediate type B, and one out of three cases of malignant trophoblastic teratoma. A follow-up study on one patient with malignant teratoma of intermediate type B and on two patients with malignant trophoblastic teratoma confirmed the diagnostic and prognostic importance of estrogen determinations in patients with testicular tumours. No clear-cut picture was obtained concerning testosterone excretion and blood testosterone levels. Whereas semicastration in some patients resulted in a fall of the plasma testosterone, other cases demonstrated significant elevated blood and urine testosterone levels. The possible influence of gonadotrophins – either produced by the tumour tissue or in increased amounts by the pituitary – on the steroid hormone content is discussed. Of practical importance is the finding that, even in malignant trophoblastic teratoma, the Pregnosticon test was not always sensitive enough to give a positive reaction. On the other hand, the quantitative HCG determination in blood and urine is of considerable importance for the differential diagnosis of the respective type of tumour.