Modifications of lymphocyte serotonergic system have been described in major depression. The aim of this study was to determine new possible changes of this system in depression. Twenty eight patients, free of drugs, diagnosed with major depression disorder by Structured Clinical Interview for Disorders of Axis I, without medical illnesses, written consent, approved by Ethical Committees were included. Controls were 30 healthy subjects without family history of psychiatric disease. Blood monocytes were isolated with Ficoll/Hypaque, and lymphocytes by differential adhesion to plastic. Serotonin and 5-hydroxyindoleacetic acid determined by HPLC. Monocytes had higher serotonin concentrations than lymphocytes, and serotonin/5-hydroxyindoleacetic acid was lower in patients. Basal proliferation was elevated in depressed and not increased by Concanavalin A. Fluoxetine reduced basal proliferation more efficiently in patients, indicating activation of lymphocytes in depression. The number of cells expressing serotonin transporter was reduced in depressed. There were no differences in CD4+ (approximately 50%) or CD8+ (approximately 25%) lymphocytes between the groups, although CD8+ were lower in depressed, and greater number of them co-localized serotonin transporter than CD4+, which could be crucial for function in relation to serotonin and its receptors in immune cells. Lymphocytes were activated in this group of patients and fluoxetine reduced proliferation, probably being relevant for the psychopharmacological treatment of depression.