In the present work, we provide data supporting that CD70, a tumor necrosis factor (TNF)-related molecule, defined as the CD27 ligand (CD27L), may actively regulate T cell functions similarly to other members of the TNF family (i.e., CD40L and CD30L). Cross-linking CD70 with specific monoclonal antibodies (mAb) stimulated cytotoxicity and cytokine production in human T cell clones. Detection of intracellular-free calcium mobilization and mitogen-activated protein kinase phosphorylation upon mAb engagement of CD70 further supported an active signaling role for the TNF-related molecule. Similar results were obtained in the Jurkat leukaemia T cell line stably transfected with CD70; in that system, induction of Akt phosphorylation was detected, indirectly revealing the involvement of the phosphatidylinositol-3 kinase pathway. Stimulation of CD70+ Jurkat cells, with a CD70-specific mAb or with COS-7 cells transiently transfected with CD27, induced transcriptional activity detectable by different reporter gene expression systems. Altogether, our data point out that a reciprocal communication may be established between CD27+ and CD70+ cells during the immune response.