New 7-substituted camptothecin derivatives possessing anti-tumor activity with slight toxicity, represented by the general formula: ##STR1## wherein R stands for --CHO, --CH.sub.2 OR', --CH(OR').sub.2 or --CH.dbd.N--X where R' is a lower alkyl group with 1-6 carbon atoms or a phenylalkyl group with 1-3 carbon atoms in the alkylene moiety thereof, and X is a hydroxyl group or --NR.sup.1 R.sup.2 where R.sup.1 and R.sup.2 are the same or different and each represents a hydrogen atom or a lower alkyl group with 1-6 carbon atoms, or when R.sup.1 is hydrogen, R.sup.2 may be a lower alkyl group with 1-6 carbon atoms, a substituted or unsubstituted aryl group, a carbamoyl group, an acyl group, an aminoalkyl group or an amidino group, or when R.sup.1 is the lower alkyl group, R.sup.2 may be an aminoalkyl group, or R.sup.1 and R.sup.2 may be combined together with the nitrogen atom, to which R.sup.1 and R.sup.2 are bound, to form a heterocyclic group, and quaternary salts thereof. These 7-substituted camptothecin derivatives are prepared by treating 7-hydroxymethyl- or 7-dialkoxymethyl-camptothecin with a catinoid reagent, or treating 7-formyl- or 7-hydroxymethyl-camptothecin with an acid in the presence of a lower alkanol or phenylalkanol, or treating 7-formylcamptothecin or an acetal thereof with a nitrogen-containing carbonyl reagent and optionally treating the resulting product with a quaternating agent.