UDP-N-acetylgalactosamine:polypeptide N-acetylgalactosaminyltransferases (GalNAc transferases) catalyze the initial reaction in O-linked (mucin type) oligosaccharide biosynthesis. To attempt to inhibit the synthesis of O-glycan, we transfected antisense cDNA of GalNAc transferase type 1 (GalNAc-T1) into a human gastric cancer cell line, JRST. A decreased expression of GalNAc-T1 at the level of both mRNA and protein was observed in the resultant transfectants. They demonstrated a significantly increased sensitivity to NK and lymphokine-activated killer cells in vitro compared with parental cells and mock transfectants. Although there was no significant difference in in vitro cell proliferation among parental cells, mock transfectants, or antisense transfectants, the in vivo growth rate of antisense transfectants using SCID mice was clearly lower than that of parental cells and mock transfectants. Furthermore, the treatment of mice with anti-asialo-G(M1) Ab abolished this growth-inhibitory effect of antisense transfection. From these results, we conclude that antisense suppression of GalNAc-T1 could increase the sensitivity of tumor cells to NK and lymphokine-activated killer cells, suggesting that this strategy may be of use as a new immunogene therapy.