We have examined the influence of diazepam binding inhibitor (octadecaneuro-peptide, DBI33-50) on cell mediated immune responses including LPS-stimulated monocyte IL-6 secretion, PHA induced lymphocyte proliferation and NK cell function in humans. All studies were performed in vitro on isolated human peripheral blood mononuclear cells in the absence or presence of synthetic DBI33-50. It has been shown that DBI33-50, in concentration between 10(-6)-10(-8) M, enhances the LPS-induced secretion of IL-6, as determined by specific bioassay for this monokine. On the other hand DBI33-50 (10(-6)-10(-12) M), had no significant effect on either PHA-induced lymphocyte proliferation or NK cell function. This data suggests a possible immunomodulatory role for DBI33-50 as an endogenous neuropeptide, which stimulates IL-6 secretion by human monocytes.