The efficiency of cross-presentation of exogenous antigens by dendritic cells (DCs) would seem to be related to the level of antigen escape from massive degradation mediated by lysosomal proteases in an acidic environment. Here, we demonstrate that a short course of treatment with chloroquine in mice during primary immunization with soluble antigens improved the cross-priming of naïve CD8+ T lymphocytes in vivo. More specifically, priming of chloroquine-treated mice with soluble ovalbumin (OVA), OVA associated with alum, or OVA pulsed on DCs was more effective in inducing OVA-specific CD8 + T lymphocytes than was priming of untreated mice. We conclude that chloroquine treatment improves the cross-presentation capacity of DCs and thus the size of effector and memory CD8+ T cells during vaccination. Copyright © 2008, American Society for Microbiology. All Rights Reserved.