In the present study, we investigated the expression of human CD1d antigen on activated mature T cells. Expression of this glycoprotein was found to be highly regulated and dependent on PHA stimulation. Flow cytometry studies using the NOR3.2 antibody, which recognized CD1d under denaturing conditions, showed a clear increase in its expression after PHA stimulation. Expression of this molecule after PHA activation was confirmed by analysis of its corresponding transcript by RT-PCR. A single band representing mRNA for CD1d membrane isoform was observed in activated PBMC as well as in ER3 CD1D-transfected and MOLT-4, pre-T cell lines, which were used as controls. Western blot analysis revealed an activation-dependent increase in CD1d protein expression when PBMC and enriched T cells were activated for different time periods. Activation-dependent expression of CD1d antigen was also confirmed in allogenic-activated T cells, suggesting that this event could have biological significance. Finally, immunocytochemical studies showed the presence of this protein at the plasma membrane accompanied by a cytoplasmic and perinuclear distribution. Results presented herein provide the first experimental evidence showing that CD1d antigen is present on circulating, activated T lymphocytes, suggesting that its expression is dependent on the activation state of the cells. Elucidation of the molecular mechanisms implicated in the activation-dependent expression of this nonclassical antigen will provide new insights into the understanding of antigen presentation and immune regulation.