It has been previously shown that antibodies contained in human plasma directed towards the Coxsackievirus B4 (CVB4)E2 capsid protein VP4 can enhance the CVB4E2-induced production of IFN-alpha by peripheral blood mononuclear cells (PBMC). The aim of this study was to produce a VP4 fusion protein to investigate the role of the internal capsid protein VP4 and anti-VP4 antibodies in the plasma-dependent enhancement of CVB4E2 infection of PBMC. A fusion protein MBPVP4 containing the VP4 insert of CVB4E2 and a control fusion protein MBP-beta-gal-alpha, were produced in Escherichia coli K12 TB1. The CVB4E2 infection of PBMC was quantified by using a real time PCR method amplifying CVB4E2-negative strand RNA. IFN-alpha concentrations in culture supernatants were assayed by DELFIA. MBPVP4 but not MBP-beta-gal-alpha, preincubated with plasma inhibited the plasma-dependent enhancement of CVB4E2-induced production of IFN-alpha by PBMC. Human plasma samples, antibodies contained in plasma eluted from MBPVP4-coated plates, but not from MBP-beta-gal-alpha-coated plates, incubated with CVB4E2 enhanced the infection of PBMC and the production of IFN-alpha by infected cells. Together our results show that VP4 and anti-VP4 antibodies play a role in the plasma-dependent enhancement of CVB4E2 infection of PBMC.