Depressed mood has been associated with reduced natural killer cell activity (NKCA). Further, amelioration of depressive symptoms by pharmacotherapy has resulted in augmented NKCA. Serotonin, an indoleamine implicated in the pathophysiology of affective disorders, enhances NKCA in vitro and lymphocytes possess serotonin transporters and receptors. The present study evaluated NKCA in depressed outpatients before and during treatment with the selective serotonin reuptake inhibitor (SSRI) fluoxetine (Prozac(R)). Further, the SSRIs, fluoxetine and paroxetine (Paxil(R)), were also incubated in vitro with lymphoid cells to evaluate possible direct effects of SSRIs on NKCA. Depressed outpatients were administered fluoxetine (20 mg/day) for 4 weeks. NKCA and severity of depression were evaluated at weeks 0, 1, 2, and 4. Serum concentrations of fluoxetine and norfluoxetine were obtained as well. Mononuclear cells obtained from nonpatient volunteers were incubated with pharmacologic concentrations of fluoxetine or paroxetine and NKCA measured with a standard chromium release assay. Fluoxetine treatment resulted in decreased symptoms of depression and increased serum concentrations of fluoxetine and norfluoxetine. Further, fluoxetine treatment was associated with augmented NKCA in a subgroup of depressed outpatients exhibiting low NKCA at baseline. Fluoxetine had no effect on NKCA in depressed individuals exhibiting high NKCA at baseline. Incubation of mononuclear cells with fluoxetine and paroxetine augmented NKCA in vitro. The enhancing effects of antidepressants on NKCA in vivo and in vitro indicate a possible direct drug interaction with lymphoid cells during pharmacotherapy, suggesting that pharmacologic treatment of depression may result in enhanced immune competence as indexed by enhanced NKCA and that NKCA could be pharmacologically augmented with antidepressants in individuals with compromised immune function.