The use of fluorescence probes for in-vivo diagnostics is at the forefront of medical science. To transition this technique into the clinical environment, quantitative spectral analysis and knowledge of the cellular interactions of the marker probes is vital. Furthermore, fluorescence intensity and lifetime changes, as a function of physiological environment, represents a diagnostic opportunity. A new class of polyazamacrocyclic chelates of Terbium have been identified with rich spectroscopic properties. These chelates are tissue selective, have fluorescence lifetimes on the order of milliseconds, sharply spiked emission spectra (< 15 nm FWHM), large Stokes shifts (> 280 nm), good water solubility and high quantum yields (approximately 0.6 for PCTMB). We will present our in-vitro and in-vivo spectroscopic evaluation of the chelates. In addition to the spectral investigations, results from cellular binding specificity studies using Sprague-Dawley rats with UMR 108 osteosarcomas will be presented. The potential to use the Tb(III) chelates as neoplastic tissue markers will be discussed.