Bleomycetin, an antitumor antibiotic, was subjected to chemical modification by the C-end fragment i.e. the residue of 3-[(4-aminobutyl)amino]propylamine (spermidine++) with acylation, carbamoylation and reducing alkylation, which yielded its new semisynthetic derivatives. The use of physicochemical methods showed that the chemical modification involved the primary and secondary amino groups++ of spermidine++ and gave rise to N,N'-diacyl, N,N'-dicarbamoyl and N,N'-dialkyl bleomycetins. The biological properties of the derivatives, i.e. their cytotoxic activity, acute and pulmonary toxicities were studied. The transformation of bleomycetin by the C-end fragment lowered the antibiotic toxicity and was believed to be a promising approach to modifying its molecule.