Anti-extractable nuclear antigen (ENA) antibodies were assayed by counter immunoelectrophoresis (CIE) and immunoblotting in patients with systemic lupus erythematosus (SLE). We found the two methods showed good concordance rates, the lowest being 67% for anti-SS-A. Immunoblotting was more sensitive in detecting anti-Sm, anti-SS-B and anti-PCNA (proliferating cell nuclear antigen); CIE was more sensitive for anti-nRNP and anti-SS-A. Overall, the prevalence of these anti-ENA antibodies in SLE was increased by 9-20% if immunoblotting was used in addition to CIE. Sera specific for the 52 kDa peptide of the SS-A antigen (anti-52kDa SS-A) were better detected by immunoblotting. Anti-PCNA antibody was found in 6.3% of SLE patients and was associated with active disease and hemolytic anemia. The positive rate of anti-Sm was 9% by CIE and 23.7% by immunoblotting and this antibody was a specific marker for SLE using either method. It was concluded that using immunoblotting in addition to CIE, the overall sensitivity of detection of anti-ENA antibodies in SLE was increased and clinically useful antibodies such as anti-52kDa SS-A and anti-PCNA could be detected.