Germinal center dendritic cells (GCDCs) have been identified as CD11c(+) CD4(+) CD3(-) cells located in GCs with the ability of inducing marked proliferation of allogenic T cells. Using immunofluorescence techniques, we have observed that this CD11c(+) CD4(+) CD3(-) immunophenotype identified GCDCs but also a subset of extrafollicular DCs. By flow cytometry, we were able to discriminate the GCDCs (CD11c(high) CD4(high) lin(-)) from the other tonsil DCs. By immunofluorescence and flow cytometry, we found that dendritic cells of germinal centers express more intracellular adhesion molecule-1 (ICAM-1) (CD54) than extrafollicular dendritic cells. Proliferation of peripheral blood mononuclear cells (PBMCs) induced by coculture with purified CD11c(+) CD4(+) CD3(-) DCs was reduced by addition of blocking anti-CD54 antibodies. In summary, distinct levels of ICAM-1 expression allow the distinction between GCDCs and extrafollicular DCs, and cellular interactions mediated by CD54 are likely to play a role in the capacity of GCDC to stimulate allogenic PBMC proliferation.