Acute morphine impairs innate and acquired immunity. The mechanisms involved in immunosuppression have not been well defined yet. The transcription factor NF-kappaB is a central regulator of immunity, and of the NF-kappaB family, RelB is particularly involved in the expression of genes important in immune responses. We investigated the involvement of RelB in morphine-induced immunosuppression in mice deficient for the RelB factor. RelB-/- mice and wild-type (WT) controls were injected s.c. with morphine 20 mg/Kg, and 1 h later, immune parameters were evaluated. Morphine significantly reduced macrophage production of the proinflammatory cytokines IL-1beta, TNF-alpha, and IL-12 in WT animals, and the drug failed to diminish the production of these cytokines in the RelB-/- mice. In contrast, the anti-inflammatory cytokine IL-10 was similarly affected in the two strains. Macrophage NO production was modulated by morphine in WT animals only, and morphine similarly decreased macrophage chemotaxis in the presence or in the absence of RelB. When Th1 and Th2 cytokines were evaluated, we observed a clear morphine-induced reduction of IL-2 and IFN-gamma production by WT splenocytes, whereas no effect of the drug was observed in RelB-/- mice. On the contrary, the production of the Th2 cytokines IL-4 and IL-10 was lessened to the same degree by morphine in WT and RelB-/- mice. In conclusion, our data suggest that RelB is an important target for morphine modulation of proinflammatory and Th1 cytokines. They also indicate that morphine uses multiple intracellular pathways to exert its generalized immunosuppression.