Ross River virus (RRV) is the aetiological agent of epidemic polyarthritis (EPA) a predominantly rheumatic disease afflicting up to 5000 Australians annually. We show here for the first time that macrophages can be productively infected by RRV. Subneutralizing titres of anti-RRV IgG (but not IgM) also showed classical antibody-dependent enhancement (ADE) of RRV infection in macrophage and monocyte cell lines. No correlation between development of EPA and the pre-existence of ADE titres was apparent, nor could sera raised against a related arbovirus, Barmah Forest, enhance RRV infection. Tumour necrosis factor-alpha, implicated in the immunopathogenesis of rheumatoid arthritis, was not secreted by RRV-infected monocytes or macrophages. Macrophage cell lines infected with RRV were, however, capable of producing virus for over 50 days. RRV-induced arthritis may therefore be due to the persistent productive infection of macrophages, perhaps established by a brief period of ADE early in infection.