An antitumor 5-nitrobenzo[de]isoquinoline-1,3-dione of the formula I ##STR1## where n is 1 or 2, R.sup.1 and R.sup.2 are identical or different and are each hydrogen, C.sub.1 -C.sub.6 -alkyl, C.sub.1 -C.sub.6 -hydroxyalkyl, pyrrolidinyl, morpholino, piperidinyl or piperacinyl and R.sup.3 and R.sup.4 are identical or different and are each hydrogen, C.sub.1 -C.sub.6 -alkyl, C.sub.1 -C.sub.6 -acyl, C.sub.2 --C.sub.7 -alkoxycarbonyl, ureyl, aminocarbonyl or C.sub.2 -C.sub.7 -alkylaminocarbonyl, and its salts with physiologically tolerated acids, are described.