The invention relates to a selenium compound. Said selenium compound has formula (I), where R1=alkyl R2=H, R4C(=0), R4OC(=0), a-aminoacyl, CH3SeCH2CH2CH(NH2)C(=0), CH3SeCH2CH2CH(OH)C(=0) X=OH, OR3, NH2, NR4R5, α-amino acid, CH3SeCH2CH2CH(COOH)NH—, CH3SeCH2CH2CH(COOH)0- R3=alkyl R4=alkyl, aryl R5=H, alkyl, aryl R4 and R5 which can together form a 5- or 6-membered cycloalkyl radical which can comprise a heteroatom provided that when X=NH-terbutyl, R2≠C(=0)CH3. Said compound can be used as a pharmaceutical substance, in particular as an antitumour substance.