A patient with an anaplastic carcinoma of the gallbladder accompanied by marked neutrophilia was found to have high colony-stimulating factor (CSF) activity in the urine and serum. The supernatant of the cultured tumor cells (6th passage) showed high and specific CSF activity. These specimens induced the formation of granulocyte colonies, macrophage colonies or granulocyte-macrophage mixed colonies by both human and C57BL mouse bone marrow cells in soft agar. These results suggest that this tumor is a human granulocyte-macrophage (GM)-CSF producing tumor.