To elucidate the difference between scirrhous and non-scirrhous gastric carcinomas, we examined the expressions of TGF-β, procollagen type I and type III in 7 gastric carcinoma cell lines and 37 gastric carcinoma tissues, and also examined the effect of TGF-β on the expression of procollagen mRNA by TMK-I cells. TGF-β mRNA was detected in all the tumors examined in vivo and in vitro. Interestingly, 9 (90%) of 10 scirrhous gastric carcinomas revealed higher levels of TGF-β mRNA than normal tissues, while 8 (38%) of 21 well-differentiated adenocarcinomas had higher TGF-β mRNA levels than normal tissues. As for procollagen mRNA, most of the human gastric carcinoma cell lines expressed type-l procollagen mRNA and MKN-I expressed type-Ill procollagen mRNA. Furthermore, procollagen type-l mRNA accumulation in TMK-I cells was increased by exogenous TGF-β. Most of the tumor tissues from surgical specimens expressed higher procollagen mRNA than normal tissues. These results indicate that TGF-β produced by carcinoma cells might stimulate collagen synthesis not only by fibroblasts but also by carcinoma cells themselves, leading to diffuse fibrosis of scirrhous gastric carcinomas.