Feeder activity of bone marrow adherent cells and peritoneal macrophages is decreased and colony-stimulating and erythropoietic activity of unfractionated bone marrow increased in highly leukemic AKR/JY mice in comparison with CBA/CaLac mice. At the same time maturation of hemopoietic precursor cells in AKR/JY mice is delayed compared to controls. This indicates compensatory activation of nonadherent elements of the hemopoiesis-inducing microenvironment against the background of suppressed activity of adherent elements. Hence, leukemogenic virus produced a systemic damage to target cells (e.g. mononuclear phagocyte system), which probably represent a mechanism of leukemic transformation in AKR/JY mice.