Human embryonic kidney cells (293) were transfected with a construct containing human factor X cDNA and selected for G418 resistance. The level of expression of recombinant factor X in serum-free medium was 4 to 5 mu g/ml. Purified recombinant factor X had a molecular size identical to that of normal plasma factor X : Amino-terminal sequencing revealed normal processing cleavages. The r-carboxy Glu and beta-OH Asp content of the recombinant factor X was close to 90% of the expected levels of these post-translational residues. The specific activity of recombinant factor X was about 95% of that of plasma factor X in three plasma-based clotting assays : This report demonstrates that 293 cells can produce a high level of biologically active factor X and describes a visual criterion for verifying the transfection process.