Objective. To assess the phospholipid specificity and immunoglobulin isotype of antiphospholipid (aPL) antibodies in patients with acute parvovirus B19 infection.Methods. Specificity of aPL and isotype distribution in the negatively charged phospholipids, cardiolipin and phosphatidyl serine, and in the neutral phospholipid, phosphatidyl ethanolamine, were measured in the sera of patients with acute parvovirus B19 infections (n = 12), in those with other acute viral infections (n = lo), and in those with syphilis (n = 15) by enzymelinked immunosorbent assays. The dependence of anticardiolipin (aCL) binding on the presence of p2-glycoprotein I (p2-GPI) as a binding cofactor was assessed in these same groups, and was compared with sera from systemic lupus erythematosus (SLE) patients (n = 11) with raised aCL antibody reactivity.Results. Antibodies against any of the 3 phospholipids were found in all 3 groups of patients with infections (B19 in 11 of 12 patients, other viral infections in 8 of 10 patients, and syphilis in 14 of 15 patients). B19 patients' sera contained predominantly IgG antibodies against the negatively charged phospholipids, cardiolipin and phosphatidyl serine, and differed in their specificity and isotype distribution from those found in the other 2 patient groups. B19-associated aCL increased their binding to antigen in the presence of &GPI as a binding cofactor, similar to aCL found in Conclusion. The results show the remarkable similarity in specificity of aPL antibodies between B19-infected patients and SLE patients, and raise the question of whether parvovirus infection may be a trigger for the development of aPL antibodies in autoimmune diseases.Human infection with parvovirus B19, a small, single-stranded DNA virus, is associated with a variety of clinical manifestations, including rash, arthralgia, thrombocytopenia, leukopenia, fetal wastage, hypocomplementemia, autoimmune hemolytic anemia, and vasculitis (1-3). In addition, infection can be associated with elevated levels of autoantibodies to nuclear components (antinuclear antibodies [ANA]), doublestranded DNA (dsDNA), lymphocyte antigens, neutrophi1 cytoplasmic antigens, and cardiolipin (1-4).The prevalence and specificities of anticardiolipin antibodies (aCL) in B19 infection are not known. Raised levels of antiphospholipid antibodies (aPL) or aCL have been reported in a variety of other infections, including chicken pox, hepatitis A, infectious mononucleosis, rubella, adenovirus, mumps, and human immunodeficiency virus (HIV) (5), and are also well recognized in syphilitic infections (6). In light of the similarity of some of the clinical features presented by parvovirus B19 infection with those of autoimmune connective tissue diseases, we were interested in determining if the aCL antibodies observed in B19 infection were similar to those found in systemic lupus erythematosus (SLE) and whether similar antibodies were found in patients with acute viral infections other than parvovirus B19. One distinguishing feature of aCL found in au...