A human lung cancer (OTUK-tumor) was transplanted serially to nude mice, which invariably developed a marked neutrophilia. In order to analyze this phenomenon, in vitro agar culture studies were carried out. A three- to fourfold increase of colony-forming units in culture was observed in the femurs of these nude mice. Moreover, the plasma of nude mice bearing this tumor, as well as the tumor extract, had a significantly higher colony-stimulating activity on mouse bone marrow cells than appropriate controls. This activity had the property to stimulate granulocyte and/or mixed cell type colonies rather than mononuclear cells. This activity was also demonstrated, in a dose- dependent way, on normal human bone marrow cells. These findings indicated that the OTUK-tumor produced human granulopoietic colony- stimulating activity, which may have stimulated granulopoiesis in vivo as well.