This study analysed the concentrations of thyroid binding globulin (TBG) in the serum as well as the level of triiodothyronine (T3), thyroxine (T4), thyreostimulating hormone (TSH), thyroglobulin (Tg) and T4/TBG coefficient, before therapy and on the fifth day of therapy in 27 patients of both sexes suffering from neoplasia. The patients were treated with cytostatic antibiotics, alkylating agents and podophyllines derivates, by protocol. Serum T3 gains in concentration already after 5 days since the beginning of treatment (from 1.65 +/- 0.20 to 1.73 +/- 0.18 nmol/l), but it is retained within physiological ranges as well as TSH, rT3 and Tg. Thyroxine remains within the permitted ranges, but slightly decreased after therapy (64.00 +/- 11.00 nmol/l, mean +/- SEM) in comparison to the value before (71.00 +/- 19.00), p(F) less than 0.1. A reduction of concentrations in total serum proteins at an average 7.88% was found. The level of inter-alpha globulin, TBG, in patients with proved malignancy is low before therapy (14.97 +/- 4.73 micrograms/l, mean +/- SEM, in males and 14.83 +/- 3.70 in females) but, with application of cytostatics it decreases considerably to the level of 11.68 +/- 4.46 (p less than 0.05) in males and to 13.68 +/- 3.89 in females (NS). In view of these facts, T4/TBG coefficient remains normal and contributes to the maintenance of euthyroid gland function.