Expression of the human cyclooxygenase 1 (COX-1) in Trichoplusia ni BTI Tn 5B1-4 cells transformed with cDNAs encoding ${eta}1$,4-galactosyltransferase (GalT) and $Gal{eta}1$,4-GlcNAc ${alpha}2$,6-sialyltransferase (ST6Gal) was examined. Southern blot analysis indicated that the glycosyltransferase genes were integrated into the Tn 5B1-4 cell genome. A lectin blot analysis also indicated that the recombinant COX-1s from the Tn 5B1-4/COX-1/GalT-ST cells contained the glycan residues of ${eta}1$,4-linked galactose and ${alpha}2$,6-linked sialic acid. The specific peroxidase activity of the recombinant sialylated COX-1 from Tn 5B1-4/COX-1/GalT-ST cells was 23,930 units/mg, indicating an increase of approximately 17% compared with the non-sialylated control (20,500 units/mg) from the Tn 5B1-4/COX-1 cells.