Abstract Respiratory infections can cause significant morbidity and mortality in prematurely delivered babies. The increased susceptibility of preterm babies to infection is related to immaturity of the immune system. The Toll-like receptors (TLR) are expressed by antigen-presenting cells and play a key role in recognition of pathogens and regulation of immune response. However, their role in preterm babies is not well-understood. In this study, we investigated the expression of TLR2 and TLR4 proteins in lung tissues of fetal (delivered at 125, 140, 175 days of gestation age, dGA; complete term 185 days) and naturally-infected, prematurely delivered (125dGA ventilated with PRN O2) baboons. The expression of TLR2 and TLR4 was studied at protein level by Western blotting using anti-human-TLR2 and TLR4 antibodies. We found that the expression of TLR2 and TLR4 protein increased with the gestational age; the expression was found low in lung tissues of 125dGA baboon which increased to adult level at 175dGA. But in the lung tissues of naturally-infected preterm baboons, the protein levels of both TLR2 and TLR4 were significantly increased as compared to non-infected controls. Our results suggest that the expression of TLR2 and TLR4 is developmentally regulated and it may play a significant role in regulation of immune responses during infection in preterm babies. Research Support by American Lung Association