Dynorphin 1-17 (DYN 1-17) is biotransformed rapidly to a range of fragments in rodent inflamed tissue with dynorphin 3-14 (DYN 3-14) being the most stable and prevalent. DYN 1-17 has been shown previously to be involved in the regulation of inflammatory response following tissue injury, in which the biotransformation fragments of DYN 1-17 may possess similar features. This study investigated the effects of DYN 3-14 on lipopolysaccharide (LPS)-induced nuclear factor-kappaB/p65 (NF-κB/p65) nuclear translocation and the release of pro-inflammatory cytokines interleukin-1beta (IL-1β) and tumor necrosis factor-alpha (TNF-α) in differentiated THP-1 cells. Treatment with DYN 3-14 (10nM) resulted in 35% inhibition of the LPS-induced nuclear translocation of NF-κB/p65. Furthermore, DYN 3-14 modulated both IL-1β and TNF-α release; inhibiting IL-1β and paradoxically augmenting TNF-α release in a concentration-independent manner. A number of opioids have been implicated in the modulation of the toll-like receptor 4 (TLR4), highlighting the complexity of their immunomodulatory effects. To determine whether DYN 3-14 modulates TLR4, HEK-Blue™-hTLR4 cells were stimulated with LPS in the presence of DYN 3-14. DYN 3-14 (10μM) inhibited TLR4 activation in a concentration-dependent fashion by suppressing the LPS signals around 300-fold lower than LPS-RS, a potent TLR4 antagonist. These findings indicate that DYN 3-14 is a potential TLR4 antagonist that alters cellular signaling in response to LPS and cytokine release, implicating a role for biotransformed endogenous opioid peptides in immunomodulation.