A cell line (KSNY) in vitro, which produces colony-stimulating activity (CSA) for human and mouse marrow cells, has been established. A biopsy was performed on the tumor mass of a lung cancer patient who had developed extensive leucocytosis. A piece of the tumor was transplanted to a nude mouse. The seconarily transplanted mice, in turn, developed extensive leucocytosis. The mouse tumor was then removed and placed in culture bottles. To date, the KSNY cells have been maintained in vitro continuously for 15 months. By the use of a methylcellulose bone-marrow colony-formation technique, a high level of CSA in the supernatant of the tumor cell culture was recognized. Doubling time of the cell line is 46 hr. The modal chromosome number is 52, ranging from 45 to 106.