The interaction of four representative polyanions with parathyroid hormone (PTH) residues 1-84 has been investigated utilizing a variety of spectroscopic and calorimetric techniques. Each of the polyanions employed demonstrate enthalpically driven binding to PTH (1-84) with significant affinity. The polyanions heparin, dextran sulfate, phytic acid, and sucrose octasulfate induce alpha-helical structure in PTH to varying extents depending on the ratio of polyanion to protein employed. Intrinsic and extrinsic fluorescence spectroscopy suggests significant protein tertiary structure alteration upon polyanion binding. Although structural modification occurred upon polyanion binding, PTH colloidal stability was increased depending on the ratio of polyanion to protein used. Nevertheless, the bioactivity of PTH in the presence of various ratios of heparin was not altered. The potential biological significance of PTH/polyanion interactions is discussed.