The sustained release nature of ofloxacin microspheres--to eradicate bacterial biofilm associated with chronic infections from sensitive strains of bacteria--was determined both in vitro and in vivo. Ofloxacin microspheres were prepared by emulsion solvent evaporation procedure using poly(glycolic acid-co-dl-lactic acid) (PLGA) as the biodegradable polymer. The microspheres were characterized by scanning electron microscopy, in vitro release in an incubator, and in vivo release in the rat subcutaneous model. The microspheres were highly spherical with a very smooth surface. Approximately 45% of the drug was released from microspheres in sizes of 125-250 microns and 250-425 microns in 2 days compared with approximately 22% from microspheres of size range 37-125 microns indicating that surface area of the microspheres did not control the kinetics of in vitro release. However, about 96% of the drug was released from the three different size ranges in 35 days. The in vitro release profile of microspheres of size range 125-250 microns is not significantly different from microspheres in sizes of 250-425 microns. The peak plasma level of ofloxacin in animals that received the drug suspension occurred within 2 hr and was higher than that of the microspheres that occurred by the end of the second day. The plasma of animals that received the free drug was depleted of ofloxacin by the end of the first day, but the drug was sustained above 0.5 microgram/mL in the plasma of animals that received the microspheres for about 3 weeks. The results suggest that biodegradable ofloxacin microspheres can be prepared that release the antibiotic in vivo for about 3 weeks. This should provide a means for continuous treatment of chronic infections in which bacterial biofilm can occur.