St. John's Wort (Hypericum perforatum; H. perforatum) is a popular herbal supplement used to treat mild to moderate depression. H. perforatum possesses serotonergic properties such as inhibition of serotonin (5-hydroxytryptamine; 5-HT) reuptake. Serotonergic pharmacotherapy is associated with amelioration of depression as well as increases in natural killer (NK) cell activity (NKCA). Also, 5-HT and 5-HT analogs augment NKCA in vitro. Considering the serotonergic properties of H. perforatum, the effects of H. perforatum on NKCA were assessed in vitro. Mononuclear cells (MNCs) from normal donors were exposed in vitro to an extract of H. perforatum (LI160s) or established 5-HT stimulators of NKCA. After an overnight incubation, cells were washed and a standard 51Cr-release cytotoxicity assay performed to assess NKCA. LI160s at all concentrations failed to augment NKCA. However, in corroboration of previous studies, 5-HT, the selective serotonin reuptake inhibitors (SSRIs), paroxetine and norfluoxetine, and alpha-interferon augmented NKCA above control levels. Though an efficacious treatment for mild to moderate depression, H. perforatum differs from commonly prescribed serotonergic antidepressants insofar as H. perforatum fails to enhance NKCA in vitro. Therefore, the present results are consistent with pharmacologic studies indicating that H. perforatum possesses, at best, weak serotonergic activity.