Topoisomerase I (topo I) is a major systemic sclerosis (SSc)-associated autoantigen. A cDNA construct encoding full length human topo I in a recombinant baculovirus transfer vector was used to infect insect cells in culture from which recombinant protein was purified. An ELISA using recombinant protein was evaluated in 340 sera including sera from 134 patients with SSc, of whom 33 had anti-topo I antibodies detected by immunodiffusion. A high yield of pure topo I of expected molecular mass and catalytic activity was obtained. The recombinant topo I ELISA was 92% sensitive and 98% specific in detecting anti-topo I antibodies which were present almost exclusively in patients with SSc. Therefore, the potential advantages of expressing human autoantigens in eukaryotic systems for diagnostic purposes were confirmed.