New nitrosourea derivatives are provided, which possess a high inhibitory activity against the leukemia and tumors with a low toxicity and which are useful for pharmaceutical purposes. The compounds have the formula ##STR1## wherein A represents a glycosyl group, a monovalent residue of methyl glucoside or of alditol, a N-substituted carbamoyloxyalkyl group or a hydroxy-substituted cyclohexyl group when n is 1, or A represents a tetravalent residue of ribostamycin when n is 4 and R represents a lower alkyl or halo-alkyl group and are prepared by treating a compound of the formula ##STR2## with a nitrosating agent.