Dendritic cells have the remarkable property of presenting any incoming antigen(1). To do so they must not only capture antigens with high efficiency and broad specificity, but must also maximize their capacity to load class II molecules of the major histocompatibility complex (MHC) with antigenic peptides in order to present a large array of epitopes from different proteins, each at a sufficient copy number. Here we show that formation of peptide-MHC class II complexes is boosted by inflammatory stimuli that induce maturation of dendritic cells. In immature dendritic cells, class II molecules are rapidly internalized and recycled, turning over with a half-life of about 10 hours. Inflammatory stimuli induce a rapid and transient boost of class II synthesis, while the half-life of dass II molecules increases to over 100 hours. These coordinated changes result in the rapid accumulation of a large number of long-lived peptide-loaded MHC dass II molecules capable of stimulating T cells even after several days, The capacity of dendritic cells to load many antigenic peptides over a short period of initial exposure to inflammatory stimuli could favour presentation of infectious antigens.