The bacterial endotoxin LPS is a potent stimulator of monocyte and macrophage activation and has been shown to protect differentiated macrophages from de novo infection by HIV-1. However, the mechanisms of this inhibitory activity of LPS are not fully understood. We investigated the effect of LPS on the early post-binding steps of HIV-1 replication in primary macrophages. Paradoxically, when applied together with the virus, LPS stimulated entry of HIV-1 into macrophages, as judged by the amount of internalized HIV-1 RNA and p24. This stimulatory activity did not depend on receptors used for entry and did not require new protein synthesis. However, internalized viral RNA and p24 were rapidly degraded in LPS-stimulated macrophages. Surprisingly, while degradation of HIV-1 p24 in LPS-treated cells was inhibited by bafilomycin A1, HIV-1 RNA was not protected by this agent, suggesting that viral RNA is degraded by a pH-independent mechanism. These results indicate that LPS stimulates both virus uptake and virus degradation in macrophages.