Biodegradable nanospheres generated from a biocompatible polymer, poly(D,L-lactide-co-glycolide) (PLGA), have been studied extensively as implantable reservoirs for sustained-release drug delivery. PLGA-nanospheres have also been studied as vehicles to deliver antigens to phagocytes. The intracellular ar processing pathway of antigens delivered to phagocytes by PLGA particles was studied in the present study. Ovalbumin (OVA) encapsulated with PLGA (OVA-nanosphere) was efficiently captured, processed and presented on class I major histocompatibility complex (MHC-I) by dendritic cells (DCs). The MHC-I processing of OVA-nano-spheres was resistant to lactacystin, a proteosome inhibitor, and brefeldin A, which blocks anterograde transport from the endoplasmic reticulum (ER) through the Golgi apparatus. Chloroquine, which inhibits phagolysosomal enzymes by increasing ph ago lysosomal pH, inhibited MHC-I processing of OVA-nanospheres. In addition, DCs generated from TAP-/- mice were markedly suppressed in MHC-I processing of OVA-nanospheres. These results demonstrate that DCs process phagocytosed OVA-nanospheres via a vacuolar alternate MHC-I pathway for presentation of OVA peptides to T lymphocytes.