Chitosan microspheres containing oxytetracycline (OTC), an antibiotic agent, were prepared by spray hardening and interfacial acylation methods. The object of this study was to prepare oxytetracycline-containing microspheres for oral administration and injection using different molecular weight (Mw 70 000 similar to 2 000 000) of chitosan. By the spray hardening method, microspheres with particle sizes between 5 and 30 mu m could be obtained and might be suitable for intramuscular injection. On the other hand, chitosan microspheres with the ability to extend the dissolution period of oxytetracycline in low pH medium were also prepared by the interfacial acylation method. The result indicated that the releasing of oxytetracycline from various acylated chitosan microspheres was decreased with increasing the molecular weight of chitosan and would show well sustained-release property. Besides, the morphology of various microspheres and crystalline form transition of oxytetracycline were also studied using electron scanning microscope and X-ray analysis.