Abstract: β-Endorphin (β-EP) and delta opoid receptor (DOR) agonists affect immune functions such as lymphocyte chemotaxis, proliferation, and cytokine production. Recent studies indicate that both neuronal DOR and novel G-protein-coupled receptors with high affinity for β-EP and DOR agonists are expressed by mononuclear cells. In addition, proenkephalin A mRNA and enkephalin-related peptides are expressed by lymphocytes. These investigations were conducted to identify signal transduction pathways that mediate the effects of β-EP and DOR agonists on T cells. Calcium mobilization was studied because it is central to T-cell activation initiated by antigen presentation to the T-cell receptor (TCR). Using the calcium-sensitive dye Fluo-3 and flow cytofluorometry to determine the concentration of free intracellular calcium, physiological concentrations of β-EP were shown to enhance concanvalin. A (con A)-stimulated calcium mobilization by murine splenic T cells (p < 0.01). The DOR antagonist, naltrindole, inhibited this, whereas CTAP, a selective mu OR antagonist, was ineffective. In addition, N-Ac-β-EP and the μ OR agonist DAMGO, failed to mimic the effects of β-EP. Although it was less potent than β-EP, DADLE, a DOR agonist, also enhanced Con-A-induced calcium mobilization (p < 0.01). A DOR-transfected human T-cell line (DOR-Jul.1) was developed to study signal transduction. Both DADLE and the selective DOR agonist, deltorphin, rapidly increased intracellular free calcium concentrations; ED50s were 10−9 M. Pertussis toxin prevented the response, and EGTA significantly reduced it. In addition, DADLE inhibited forskolin-stimulated cAMP production (ED50: 10−11 M). These findings with normal splenic T cells and DOR-transfected T-cell line indicate that β-EP and DOR agonists affect calcium mobilization. This is likely to modulate downstream pathways that regulate T-cell activation and function.