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Notes: Summary T-cell-growth-factor (TCGF) activated peripheral blood lymphocytes (PBL), cultured for 14 days, showed killer cell activities against natural-killer resistant Daudi cells in a 4 h 51Cr-release assay. However, the effector cells obtained from patients with nonresectable carcinoma exhibited very much lower cytotoxicity to tumor cells. To analyze the mechanism of depression, we have attempted to examine suppressor cell activities of the TCGF-activated PBL. The assay for the suppressor cell activities was made by in vitro inhibition of cell-mediated cytotoxicity by incubating radiolabeled target tumor cells with lymphokine-activated killer (LAK) cells and TCGF-activated PBL. LAK cells were induced by cultivation with recombinant interleukin-2. TCGF-activated PBL, obtained from four out of ten patients with resectable carcinoma and nine out of ten patients with nonresectable carcinoma, significantly suppressed the LAK cell activities. However, this suppression was not observed in TCGF-activated PBL from ten normal healthy control subjects. TCGF-activated PBL with immunosuppressive reactivity were named lymphokine-activated suppressor (LAS) cells. To investigate the phenotypic characterization of TCGF-activated PBL, the cells were analyzed by two-color flow cytometry. TCGF preferentially expanded CD8+CD11− cells and decreased the growth of CD8+CD11+ cells in both normal healthy control subjects and gastric cancer (resectable and nonresectable) patients. Dominantly expressed CD8+CD11− cells on TCGF-activated PBL in patients — especially those with nonresectable gastric carcinoma — showed strong LAS cell activity, irrespective of the presence of killer cell activities of CD8+CD11− cells in TCGF-activated PBL from normal healthy control subjects. The results suggested the generation of CD8+CD11− LAS cells from cancer patients, and revealed that CD8+CD11− T-cells contained killer and/or suppressor cell function. In addition, it was found that the TCGF-activated PBL from gastric cancer patients were associated with an increased proportion of CD4+Leu8+, HLA-DR+CD8+ and HLA-DR+CD25+ cells.