The objective of this study was to use the whole blood lymphocyte proliferation method to assess the immunotoxic potential of the antidepressants imipramine (a tricyclic antidepressant, TCA), fluvoxamine (a selective serotonin reuptake inhibitor, SSRI) and venlafaxine (a serotonin/noradrenline reuptake inhibitor, SNRI). In addition, we used the known immunosuppressive compound dexamethasone to compare the magnitude of the inhibitory response (IC50 value) with that of the antidepressants. There was a 10-fold difference in the concentrations of the antidepressants that inhibited concanavalin A-stimulated lymphocyte proliferation. However, the inhibitory concentration of dexamethasone was almost 1,400 times lower than any of the antidepressants. The difference in the magnitude of the inhibitory response between the antidepressants and dexamethasone suggests that the antidepressant effect may be of little clinical relevance. As the functional reserve of the immune system is poorly understood, however, it remains a possibility that some antidepressants may accumulate in vivo in a sufficiently high concentration to inhibit lymphocyte proliferation. Med Sci Res 27:773-774 (C) 1999 Lippincott Williams & Wilkins.