Sera from anti-Ro(SSA) positive and negative patients with rheumatoid arthritis (RA) were compared with those from patients with primary Sjogren's syndrome (pSS) and systemic lupus erythematosus (SLE) with regard to anti-La(SSB) antibodies. Several assays for anti-La(SSB) (RNA precipitation, counterimmunoelectrophoresis, and immunoblotting) demonstrated the presence of such antibodies in selected anti-Ro(SSA) positive (10/19 in RA and 18/37 in pSS and SLE) but not in anti-Ro(SSA) negative sera. In agreement with previous reports, anti-La(SSB) antibodies from pSS and SLE patients uniformly reacted with various cellular extracts used as sources of La(SSB) antigen, including extracts from human cultured cells (HeLa), calf thymus and rabbit thymus. In contrast, while all 10 anti-La(SSB) sera from RA patients reacted with the human HeLa cell extracts, only three of them also reacted with calf and rabbit thymus extracts. These results were further supported by the analysis of a cohort of 70 consecutively selected sera (displaying monospecific anti-Ro(SSA) reactivity against calf thymus extract) from patients with various rheumatic disorders, where human-specific anti-La(SSB) were detected in two out of the five of RA patients studied.