Anti-La(SS-B) precipitin-negative sera show a restricted epitope recognition and can be easily overlooked in routine laboratory testing. We have therefore determined the prevalence of nonprecipitating anti-La(SS-B) antibodies in patients with primary Sjögren's syndrome and studied their clinical and immunological associations. Clinical details were obtained from 68 patients with primary Sjögren's syndrome, and serum samples were examined by enzyme-linked immunosorbent assay using purified recombinant La, 60-kDa Ro, and 52-kDa Ro proteins and by counterimmunoelectrophoresis. Thirteen patients (19%) were identified with anti-La antibodies which were nonprecipitating. These patients had similar clinical findings to other groups of patients with Sjogren's syndrome, but had significantly lower rheumatoid factor and serum IgG levels than patients with anti-La precipitins. None of the patients with nonprecipitating anti-La antibodies had previously contained anti-La precipitins in their sera. Furthermore, they tended to have lower levels of antibodies directed against denatured 60-kDa Ro (but not 52-kDa Ro) compared with anti-La precipitin-positive patients. Patients with Sjögren's syndrome associated with nonprecipitating anti-La antibodies represent a stable serological and clinical subset in which there appears to be limited diversification of the autoimmune response to the Ro60 and La proteins of the Ro/La ribonucleoprotein.