The influence of alginic acid, an anionic polysaccharide, on the conversion of 13-93 bioactive glass to hydroxyapatite (HA) in aqueous phosphate solution was investigated. Disks of 13-93 glass were immersed in phosphate-buffered saline (PBS) containing 0-1 weight percent (wt%) alginic acid. HA was detected on the glass surface by scanning electron microscopy, X-ray diffraction, and Fourier transform infrared spectroscopy after 2 weeks in PBS containing 0.5 wt% alginic acid, compared with 6 weeks in PBS without alginic acid. In the presence of alginic acid (0.5 wt%), the growth of the HA layer on the glass increased almost linearly with time, at a rate that was approximately six times faster than that for the system without alginic acid. The thickness of the HA layer increased with the concentration of alginic acid (0-1 wt%). Chelation of the electron-donating moieties of alginic acid polymer, such as carboxyls and hydroxyls, to the leached cations from the bioactive glass, in particular calcium ions, is suggested as a mechanism for the enhanced corrosion of the glass and, hence, the enhanced conversion to HA. The use of natural or synthetic polyelectrolytes could provide a method for manipulating the conversion rate of bioactive glass to HA.