Chronic bone marrow injury can occur as a sequela of myelonecrosis or acute myelitis, be associated with chronic conditions, or result from prolonged or recurrent exposure to a toxic agent. These chronic responses in bone marrow include chronic inflammatory responses (e.g., lymphocytosis, plasmacytosis, histiocytic proliferation, and granulomatous inflammation), secondary myelofibrosis, osteosclerosis, osteomyelitis, and serous atrophy of fat/gelatinous transformation. In general, domestic animals and avian species typically have less than 10% lymphocytes in cellular bone marrow specimens. Fibrosis of the bone marrow has been reported in various forms in laboratory animals. Myelofibrosis can be seen as a background finding in aging rats, but also a consequence of compound administration, whole body irradiation, and certain viral infections. Myelostromal proliferation has been described as a rare condition of bone marrow in the rat.