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Abstract. Objective:  A lipopolysaccharide (LPS) induced mastitis model in rats was used to study the protective effect of retinoid. Materials and methods:  Commencing at gestation day 10, retinoid (dissolved in olive oil) or an equal volume of olive oil was administered to rats daily by gavage until parturition. LPS or pyrogen-free physiological saline was inoculated into the mammary gland 72h after parturition and the rats were euthanized at 12h post-infection. Results:  Myeloperoxidase (MPO), N-acetyl -β-D- glucosaminidase (NAGase), tumor necrosis factor-alpha (TNF-α), interleukin-8 (IL-8) in mammary tissues and CD4+/CD8+ in peripheral blood were increased and serum MPO and IL-2 in mammary tissues were decreased 12h after LPS infusion. Retinoid decreased MPO, NAGase, and TNF-α in mammary tissue and increased IL-2 in serum. Four thousand and 8000 I.U/kg•d of retinoid significantly decreased the infiltration of PMNs in mammary alveoli and ameliorated the imbalance of CD4+/CD8+ in peripheral blood. Conclusion:  These results suggest that retinoid protected against LPS-induced mastitis in a rat model. [ABSTRACT FROM AUTHOR]