[en] The antitumor antibiotics bleomycins labeled with ⁵⁷Co are known to possess excellent tumor localizing properties but the rather long halflife of ⁵⁷Co prevents its use in clinical routine. It is therefore desirable to label cobalt-bleomycin with a more suitable radionuclide, e.g. ¹²³I. This thesis reports on further studies on cobalt-bleomycin. It appears from the studies on the structure of cobalt-bleomycin described in this thesis (Chapter B), that cobalt is able to form different complexes with bleomycin (the forms I and II). The difference in structure is not clear, but the biological behavior of both forms is studied (Chapter C). In Chapter D the iodination of cobalt-bleomycin is described. Iodination of free bleomycin yields a product with bad tumor localizing properties, and straight-on iodination of cobalt-bleomycin is prevented by the presence of cobalt. To retain the good tumor-localizing properties of cobalt-bleomycin, possibilities were explored to incorporate the iodine in the terminal amine (a side chain, not involved in complexation). Alkylation of cobalt-bleomycin demethyl A₂ with N-bromoacetyl-3-iodoaniline yielded a product; unfortunately this product possessed bad tumor localizing properties and moreover, was not stable in vivo. The structure of a possibly successful iodinated cobalt-bleomycin is outlined but could not be realized during this research. (Auth.)