The present invention relates to fluorescent cobalamins and uses of these compounds. More particularly, this invention relates to fluorescent cobalamins that comprise a fluorescent, phosphorescent, luminescent or light-producing compound covalently linked to cobalamin. These fluorescent cobalamins can be used as diagnostic and prognostic markers (a) to distinguish cancer cells and tissues from healthy cells and tissues and (b) to determine if an individual will respond positively to chemotherapy using cobalamin-therapeutic bioconjugates.