Mouse malignant T-lymphoma CS21 cells can grow when cocultured with CAl2 lymph node stromal cells, but they undergo apoptotic cell death with DNA fragmentation when separated from CA12 stromal cells. In the course of examining the effects of the soluble factor (s) secreted by CAl2 stromal cells on CS2l cell growth, we found that thiols including cysteine promoted CS2l cell growth. P388 cell growth was also promoted by various thiols. These results suggest that thiols including cysteine participate in CA12 and P388 cell growth.