An 1194-nucleotide complementary DNA clone, FM1, encoding a human high-mobility group-1 protein (HMG-1) was isolated from a well-differentiated human gastric-carcinoma cell line complementary DNA library by a differential screening method. FM1 is similar to the published human HMG-1 in mature protein, with only 3 different codons at positions 11, 149, and 190. We analyzed 33 gastric and colorectal adenocarcinomas for expression of the FM1 gene. Northern-blot analysis revealed that all of the cancers expressed FM1 at a higher level than in corresponding non-cancerous mucosa, with 2 transcripts of approximately 1.4 and 2.4 kilobases. The FM1 expression level in the non-cancerous tissues increased with the depth of accompanying cancer invasion. Only 18.2% of well-differentiated cancers showed a higher expression level in corresponding non-cancerous tissues, whereas the expression in corresponding non-cancerous tissues was significantly higher in moderately (60%) and poorly differentiated (83.3%) cancers. In situ hybridization demonstrated the location of FM1 mRNA in well- and poorly differentiated gastric-cancer cells as well as in non-cancerous tissue adjacent to poorly differentiated gastric cancer, but no hybridization was detected in normal epithelial cells adjacent to well-differentiated gastric cancer. These findings may provide new information on HMG-1 mRNA expression in human gastrointestinal cancer and suggest a correlation between FM1 mRNA expression to the differentiation and the stage of human gastrointestinal adenocarcinomas.