Despite decades of intensive research, the development of a diagnostic test for major depressive disorder (MDD) had proven to be a formidable and elusive task, with all individual marker-based approaches yielding insufficient sensitivity and specificity for clinical use. In the present work, we examined the diagnostic performance of a multi-assay, serum-based test in two independent samples of patients with MDD. Serum levels of nine biomarkers (alpha1 antitrypsin, apolipoprotein CIII, brain-derived neurotrophic factor, cortisol, epidermal growth factor, myeloperoxidase, prolactin, resistin and soluble tumor necrosis factor alpha receptor type II) in peripheral blood were measured in two samples of MDD patients, and one of the non-depressed control subjects. Biomarkers measured were agreed upon a priori, and were selected on the basis of previous exploratory analyses in separate patient/control samples. Individual assay values were combined mathematically to yield an MDDScore. A 'positive' test, (consistent with the presence of MDD) was defined as an MDDScore of 50 or greater. For the Pilot Study, 36 MDD patients were recruited along with 43 non-depressed subjects. In this sample, the test demonstrated a sensitivity and specificity of 91.7% and 81.3%, respectively, in differentiating between the two groups. The Replication Study involved 34 MDD subjects, and yielded nearly identical sensitivity and specificity (91.1% and 81%, respectively). The results of the present study suggest that this test can differentiate MDD subjects from non-depressed controls with adequate sensitivity and specificity. Further research is needed to confirm the performance of the test across various age and ethnic groups, and in different clinical settings. © 2013 Macmillan Publishers Limited.