One hundred and twenty-four specimens (72 primaries and 52 metastases) in 107 patients with gastric cancer were tested using the human tumor clonogenic assay (HTCA). Assays for 64 specimens (52%, 33 primaries and 31 metastases) were considered evaluable. The clonogenicity of the primaries was less than that of the metastases (P less than 0.01). In the primary specimens, well-differentiated tumors were more highly clonogenic than poorly differentiated or signet ring cell tumors (P less than 0.05). Viable malignant cells and clonogenic cells were abundant in the primary tumors with mucosal (m) and submucosal (sm) invasion, i.e., in early gastric cancers by Japanese criteria, and were reduced in numbers as the primary tumors advanced (P less than 0.05). In vitro chemosensitivity corresponded well to the clonogenicity of the primary tumors stratified by the degree of tumor invasion. At the early phase of primary tumor progression and metastasis, clonogenicity and chemosensitivity were high. Aggressive chemotherapy with sensitive drugs after surgery may improve the clinical results for the gastric cancer patients with early phase of metastases.