Cloned high-metastatic Lewis lung carcinoma. A11 cells were retrovirally transduced with either granulocyte macrophage-colony stimulating factor (GM-CSF) or beta-galactosidase gene and examined for their tumorigenicity. GM-CSF-engineered A11 cells produced a much higher amount of GM-CSF than the parental and control cells. Unexpectedly, GM-CSF-engineered A11 cells grew more rapidly than the control cells, while in vitro growth rates of these cells were almost the same. The enhanced tumor growth seemed to be unique to GM-CSF among various cytokines, because interleukin 2 (IL-2), interleukin 4 (IL-4) and interleukin 6 (IL-6) producer cells exhibited suppressed tumor growth.