Parvovirus B19 infection diagnosis is based on antibodies detection against structural protein VP-1 and VP-2. The third major viral protein, non- structural protein (NS-1), has been cloned and expressed; the literature reports the detection of antibody anti-NS-1 in three immunodeficient patients with persistent infection and in three arthritic patients. On peripheral blood of 10 patients with chronic arthritis, using an immunoblot (Mikrogen, Munich-Germany), we found antibodies against NS-1 in 4 of 10 patients with chronic arthropathy associated with serologic evidence of Parvovirus B19 infection; the detection of NS-1 antibodies has been always negative when carried out at the onset of the arthritis. We believe that antibodies anti NS-1 could be an useful marker of Parvovirus B19-associated chronic arthritis.