The existence of major histocompatibility complex (MHC) class II molecules in lipid rafts has been described in dendritic cells (DC); however, the importance of rafts in T-cell activation has not been clarified. In this study, the distribution of the lipid raft components (CD59 and GM1 ganglioside) in human monocyte-derived DC was investigated. DC had an even distribution of these components at the cell surface. In addition, raft-associated GM1 ganglioside colocalized with cross-linked MHC class II. This implies coaggregation of raft components with these MHC molecules, which may be important in the interaction between T cells and antigen-presenting cells. In studies carried out to investigate the effect of the DC : T-cell interaction on raft distribution, we found a clustering of the lipid raft component CD59 on DC at the synaptic interface, with associated activation of the interacting T cell. In an antigen-specific response between DC and CD4+ T-cell clones, disruption of lipid rafts resulted in inhibition of both CD59 clustering and T-cell activation. This was most pronounced when limiting amounts of cognate peptide were used. Together, these data demonstrate the association of MHC class II with lipid rafts during DC : T-cell interaction and suggest an important role for DC lipid rafts in T-cell activation.