SERPINB3 is a serine protease inhibitor with pleiotropic functions. It is involved in several physiological and pathological processes, where it appears to exert antiapoptotic effects. Little is known about its expression on immune system cells, the major players in mechanisms of viral defense and autoimmune disorders. The aim of this study was to characterize the expression of SERPINB3 on the surface of peripheral blood mononuclear cell subsets in both normal subjects and in patients with chronic viral infections and autoimmune diseases. Sixty-two patients were analyzed by flow cytometric analysis, including 45 with hepatitis C virus (HCV)-related chronic liver disease and 17 with systemic lupus erythematosus (SLE). SERPINB3 was expressed on B lymphocytes in 79% of the controls, in 32% of the HCV-infected patients and in none of the SLE patients. Surface localization of SERPINB3 was confirmed by confocal microscopy. SERPINB3 positivity was associated with CD27 reactivity (r = 0.98), but not to other activation molecules (CD69, CD71, CD86 and CXCR3). SERPINB3 is physiologically expressed on the surface of CD27(+) B lymphocytes, but its expression is reduced in HCV viral infection and not detectable in SLE patients. These results may suggest a role for SERPINB3 in B-cell defects typically found in viral infections and autoimmune disorders.