Non-steroidal anti-inflammatory drugs (NSAIDs) are frequently associated with immune-mediated hypersensitivity reactions. The NSAID diclofenac is associated with several distinct allergic and autoimmune-like reactions including anaphylaxis, idiosyncratic hepatotoxicity and autoimmune hemolytic anemia. The aim of this study was to examine the immunostimulating potential of diclofenac in the direct popliteal lymph node assay (PLNA) and reporter antigen PLNA. In BALB/c mice, diclofenac caused dose-dependent increases in PLN weight and PLN cellularity in the direct PLNA; 0.25 mg was non-immunostimulating whereas 0.50-1.00 mg caused a significant PLN reaction. In the direct PLNA, diclofenac also increased the percent of T cells in the PLN with activated phenotypes (CD44(high)CD62L(low) and CD44(high)CD62L(high)). Finally, the magnitude of the diclofenac-induced direct PLN reaction was significantly reduced when the assay was conducted in T-cell-deficient mice. When co-injected with the reporter antigen TNP-Ficoll (trinitrophenyl Ficoll), 0.50 mg diclofenac caused significant increases in PLN weight, PLN cellularity, and induced IgM and IgG(1) anti-TNP antibody forming cells (AFCs) in the PLN. In a final set of studies, a TNP-OVA PLNA was conducted using diclofenac, phenobarbital (negative control) and streptozotocin (positive control). As expected, phenobarbital (1.00 mg) failed to cause an increase in PLN cellularity or induce AFCs in the PLN. Streptozotocin (1.00 mg) caused significant increases in PLN cellularity, IgM AFCs, and selectively induced IgG(2a) and IgG(2b) AFCs against TNP-OVA. Likewise, diclofenac caused dose-dependent increases (0.25-1.00 mg) in PLN cellularity and IgM AFCs. However, in contrast to streptozotocin, diclofenac caused a selective dose-dependent increase in both IgG(1) and IgE AFCs. Finally, an increase in the intracellular level of IL-4, but not INFgamma, was detected in CD4(+) PLN cells following the injection of diclofenac mixed with TNP-OVA. Collectively, these data suggest that diclofenac: (i) induces a T-cell-dependent direct PLN reaction that; (ii) provides non-cognate help for IgG AFC production when co-injected with TNP-Ficoll, possibly through the formation of neo-antigens; and (iii) possesses intrinsic adjuvant activity that selectively induces IL-4 mediated production of IgG(1) and IgE against co-injected TNP-OVA.