A composition of a compound or derivative thereof in which at least one carbon atom of 5-aminolevulinic acid is a carbon isotope and/or a nitrogen atom in its amino group is a nitrogen isotope, and where said derivative is an ester, amide, salt, hydrate or solvate of the compound, and a diagnostically acceptable carrier. A method for detecting malignant tumors, by administering a tumor detecting effective amount to a host in need of tumor detection of a compound or derivative thereof in which at least one carbon atom of 5-aminolevulinic acid is a carbon isotope and/or a nitrogen atom in its amino group is a nitrogen isotope, and where said derivative is an ester, amide, salt, hydrate or solvate of said compound and detecting the malignant tumors using NMR.
