Interferon (IFN)-alpha treatment of hepatitis C virus (HCV) has limited efficacy and involves frequent and severe psychiatric side-effects. Prediction of risk for depression and antiviral efficacy of IFN-alpha treatment is of great importance for bot patient wellbeing and health care expense. IFN- stimulated genes (IFN-SGs) concurrently up-regulates rate-limiting enzymes of formation of neopterin, a by-product of pteridines biosynthesis, and kynurenine (KYN), a tryptophan (TRY) metabolite. We found elevated plasma levels of neopterin in patients (HCV genotypes 2 and 3) with poor anti-viral response; and association of IFN-alpha-induced depression with a high producer allele of IFN-gamma gene that encodes production of IFN-gamma, the strongest inducer of TRY - KYN metabolism.[1] We suggest that elevated KYN/TRY ratio predicts high risk of depression and low neopterin levels predict positive anti-viral response to IFN-alpha treatment. To test this hypothesis we propose to assess neopterin, KYN and TRY levels in already collected blood samples of 300 HCV participants from our completed study of IFNG (+874) T/A gene polymorphisms in IFN-alpha-induced depression. Our study will help to develop rapid and cost-effective methods predicting the major outcomes of IFN-alpha therapy (alone and in combination with ribavirin or protease inhibitors) of HCV (and other conditions treatable with IFN-alpha, e.g., melanoma, multiple sclerosis); and help to understand the role of inflammation in major depressive disorder.