Patients with chronic pain are often challenged with depression stemming from the long-term psychophysiological effects of their condition. Consequently, patients with chronic pain are often treated with morphine, which can induce immunosuppression, along with an antidepressant. The antidepressant citalopram (CTP; Sigma-Aldrich Chemical, St. Louis, MO, U.S.A.) is a serotonin reuptake inhibitor that is reported to have immunomodulatory effects. Thus, we investigated whether CTP administration impacted immunity in morphine-treated animals. Adult mice were pretreated for 7 days with either saline or CTP (10 or 30 mg/kg intraperitoneal injections twice daily), followed by subcutaneous implantation of a 25 mg morphine pellet for 48 hours. Spleen, thymus, and lymph nodes were harvested to analyze total cell numbers, relative lymphocyte populations, and lymphocyte function. In this study, CTP had no effect on either total cell counts or lymphocyte populations in the thymus. However, in the spleen, total splenocyte numbers in all CTP-treated animals displayed an increasing trend over saline-treated animals. Interestingly, although more cells were found in the spleen, distribution of splenic lymphocyte populations did not differ between treatments. Despite no increase in total cell number, a high dose of CTP (30 mg/kg) resulted in a significantly higher B cell population in the lymph nodes, while T cell and NK cell numbers were not different. CTP did not significantly reverse morphine-induced weight loss or splenic B cell antibody secretion in vitro. Additionally, CTP treatment demonstrated a slight but not significant increase in both splenic B and T cell mitogen-induced proliferation in vitro. In summary, CTP may have a specific potential in the attenuation of morphine's immunosuppressive effect by enhancing splenocyte numbers and lymph node B cell populations.