The expression of P185erbB2 and P21ras was examined immunohistochemically in normal mucosa, intestinal metaplasia, dysplasia, intestinal type and diffuse type carcinoma of the stomach. The positive rates of P185 and P21 in the intestinal type carcinoma were significantly higher than those of the diffuse type. The results suggest that the two types of gastric carcinoma differ not only in histo-morphology but also in molecular mechanism. P21 immunopositivity is unlikely a specific marker of malignancy in gastric mucosa because of no significant difference of immunoreactivity among intestinal metaplasia, dysplasia, and carcinoma of the stomach. The positive staining of cell membrane for P185 was observed specifically in intestinal type carcinoma, but was absent in all other lesions. So it appears to be a useful marker of malignant change in the gastric mucosa. Overexpression of erbB2 gene in gastric carcinoma was demonstrated both at the mRNA level by in situ hybridization and at the protein level by immunohistochemical staining.