Haematocrit and glutathione peroxidase activity in blood, as well as selenium levels in blood, erythrocytes and plasma, were determined in 15 patients during four courses of cisplatin combination treatment for testicular teratoma. The haematocrit steadily declined, necessitating frequent blood transfusions during or after treatment. For patients without blood transfusions during treatment the reduction of the haematocrit averaged 40%. Glutathione peroxidase activity in blood declined also; for patients without blood transfusion the reduction was 30%, which is fully explained by the decrease of the haematocrit. The enzyme activity per volume of erythrocytes remained constant during the treatment. Erythrocyte selenium level did not change significantly, but plasma selenium levels of all patients dropped within each course of chemotherapy, and progressively with each subsequent course. Between cycles the levels were largely restored to almost normal values. These results may be explained by a decreasing availability of selenium in the body to maintain the normal plasma level, due to increased retention of cisplatin in tissues and subsequent alteration of selenium metabolism.