The implication of opioid receptors in immune response has been studied using mu-, delta- and kappa-opioid receptor knockout mice. The mutant animals were compared to their wild-type (WT) counterparts for antibody (Ab) response to the prototype Ag keyhole limpet hemocyanin (KLH). Kappa-receptor deficient mice displayed higher Ab titers for either total Ig, IgM, IgG1 or IgG2a isotypes, whereas mu and delta animals behaved as wild-type mice. Therefore, endogenous kappa-receptor activation would tonically inhibit Ab response. Opioid receptor deficient mice were also used to investigate the immunosuppressive action of naltrindole, a delta-opioid receptor antagonist, shown earlier to inhibit graft rejection and the allogeneic mixed lymphocyte reaction (MLR) in vitro. Naltrindole and two related compounds inhibited MLR performed with lymphocytes from wild-type and delta-opioid receptor knockout mice. These compounds also suppressed MLR assayed with cells from triple mu/delta/kappa-opioid receptor mutants. We therefore demonstrate that naltrindole immunosuppressive activity is not mediated by any of the three mu-, delta- or kappa-opioid receptors, but by a target which remains to be discovered.