To evaluate the efficacy of antibiotic deliveredby polyactic acid (PLA) polymer in sinusitis, we induced maxillary sinusitis in 32 New Zealand white rabbits by occluding the sinus ostium and inoculating the sinus cavity with Streptococcus pneumoniae. The rabbits were divided into three groups consisting of group 1 (control group, 8 rabbits), which was treated only by reopening the ostium; group 2, which was treated by both reopening the ostium and injecting ampicillin intramuscularly (40 mg/kg/day in three divided doses, 12 rabbits); and group 3 (12 rabbits) in which a piece of PLA-polymer ampicillin (0.326 mg) sheet (1.5 X 1.5 cm) was placed within the sinus after re-establishing ostial patency. The light microscopic findings such as epithelial ulceration, loss of cilia, infiltration of inflammatory cells, and edema were less pronounced in group 2 and minimal in group 3. The electron microscopic findings such as swelling of mitochondria and endoplasmic reticulum and protruded cytoplasm were severest in the control group, followed by groups 2 and 3. The mucociliary transport speed measured at the medial wall of the maxillary sinus was highest in group 3. The results of this study suggest that treatment with PLA-polymer ampicillin may have a better efficacy in maxillary sinusitis than that with systemic administration of ampicillin.