We found that mononuclear phagocytes formed a distinct number of clusters and colonies on the bottom of a culture dish 7 days later but granulocytes did not, when a large number of human spleen cells were cultured in liquid medium. In all gastric cancer bearers and patients with portal hypertension operated on, however, colony formation was restricted to spleen cells from patients with advanced gastric cancer and from a group of patients with portal hypertension. These spleen cells formed mononuclear phagocyte colonies without the help of exogeneous colony stimulating factor (CSF). We further demonstrated that the colony-forming cells were glass non-adherent and nylon wool adherent, and that spontaneous colony formation required cooperation between the colony-forming cells and colony-stimulating cells adherent to a plastic surface.