The performance of rat liver and HEp-2 in the detection of antinuclear antibodies (ANA) was studied by two independent sites and compared against an ANA enzyme immunoassay (EIA) screen and EIA systems for the measurement of antibodies to double-stranded DNA (dsDNA) and ENA. Sixty-two sera from patients with connective tissue disease (CTD) and 398 from controls suffering from other disorders were included. The level of agreement was, for HEp-2 and rat liver (within one site), 82.0% (ANA positive/ANA negative) and 51.0% (ANA pattern); and for HEp2- and HEp-2 (between sites), 71.8 and 86.5%. On sera with the ANA homogeneous pattern, the measurement of anti-ENA EIA added little to the detection rate with anti-dsDNA EIA alone. On ANA speckled sera, the EIA reactivity depended on the reaction of the mitotic cells: while sera with positive mitoses reacted similarly to ANA homogeneous sera, in those with negative mitoses the measurement of anti-ENA added about 10% to the detection rate achieved with anti-dsDNA alone. The measurement of anti-Scl-70 and anti-Jo-1 did not markedly improve the positive rate with classical ENA (anti-SSA, -SSB, -Sm, and -RNP) alone, raising doubts about the cost efficiency of including these measurements in unselected sera. The ANA EIA identified patients with CTD at a rate similar to that for rat liver and HEp-2. However, up to 98% of the sera found to be negative by ANA EIA but positive by use of rat liver and HEp-2 were from controls. Thus, the ANA EIA may possible be used as an alternative screen, particularly in laboratories with a high frequency of sera from patients not suffering from CTD.