Abstract: This invention relates to a novel method for detecting cholesterol which is deposited in the bodies of mammals, which method comprises the steps of administering to a host an effective amount of a photosensitizer of at least one member selected from the group consisting of tetrapyrrole carboxylic acids, corresponding di- or tetrahydropyrrole carboxylic acids, mono-, di- or polyamides of said tetrapyrrole carboxylic acids with amino-mono- or dicarboxylic acids, and salts of the above compounds; applying light of sufficient wavelength to the area of said mammal to be examined; and observing the fluorescence emitted from the area in which cholesterol is deposited.