Abstract Opioid receptors are expressed not only on neuroendocrine cells but also on immunocompetent cells such as lymphocytes, monocytes and macrophages. μ-Opioid receptor agonists were found to exert immunosuppressive effects, whereas δ-opioid receptor agonists have been shown to act as immunostimulants. δ-Opioid receptor agonists stimulate T and B cells and activate granulocytes and monocytes, conversely, immunostimulation can be blocked by the non-peptidic δ-opioid receptor antagonist (NTI). We investigated the impact of NTI and of the two structurally related compounds HS-378 and HS-459 on degradation of tryptophan and formation of neopterin in mitogen-stimulated human peripheral blood mononuclear cells (PBMC). Both these biochemical pathways were found to be suppressed by all three opioid receptor antagonists, HS-378 and HS-459 exhibiting slightly greater potency than NTI. The suppression of tryptophan degradation suggests that the tested δ-opioid antagonists are able to influence the serotonergic system via a non-opioid action.