Bridging experiment. A serum sample with high-titre anti-dsDNA antibodies but no anti-SmD1 (amino acids 89–119) reactivity was spiked with increasing concentrations of dsDNA (plasmid, also used in dsDNA ELISA; Dr Fooke Laboratories) and incubated for 30 minutes at room temperature. The dilution series was subsequently tested for anti-dsDNA and anti-SmD1 reactivity in ELISA according to the instructions for use of the respective kit. The anti-dsDNA reactivity as determined by ELISA significantly decreased with increasing concentrations of dsDNA. No decrease in anti-SmD1 reactivity could be observed (data not shown). Error bars indicate the x-fold standard deviation of the duplicate determinations.