The significance of urokinase-type plasminogen activator (uPA) expression in gastric cancer development was tested by using a human uPA cDNA transfection approach and an in vivo severe combined immunodeficient (SCID) mouse model. The AGS gastric cancer cell line, which has urokinase-type plasminogen-activator receptor (uPAR) but lacks uPA, was transfected with a plasmid containing human uPA cDNA and injected into the backs of SCID mice. Compared with the parent AGS cells, uPA protein secretion in AGS-2-, AGS-4-, and AGS-8-transfected cells increased by 26.1-, 34.6-, and 4.8-fold, respectively (P < 0.05). mRNA expression levels of uPA in the AGS-4 clone were much stronger than those in AGS-2 and AGS-8 clones. After the cancer cells (2 x 10(6)) were injected s.c. into the SCID mice, a palpable mass was observed at the injection site at around 140 days post-injection, followed by accelerated growth of the xenograft up to 180 days post-injection only in the high uPA-producing clone (AGS-4). These results suggest that continuous and high production of uPA by tumor cells is one of the important factors reflecting the malignancy of gastric cancer cells.