Interstitial pneumonitis (IP) is one of the major complications after allogeneic bone marrow transplantation (incidence: 30 to 50%, fatality rate: 50 to 60%). Cytomegalovirus is associated with 60% of the cases. When no infectious agent can be detected, it is classified as idiopathic IP (IIP). Factors related to IIP are: remission induction chemotherapy, pretransplant conditioning regimen - usually including total body irradiation and cyclophosphamide (CY) - Graft-versus-Host disease and posttransplant immunosuppression. The influence on the lung by these various factors, separately and in combination, is presently being investigated in a rat model. Consecutive studies have been focussed on lung injury by radiation, CY and cytosine arabinoside (Ara-C). Parameters for evaluation of lung damage were lung function measurements, animal survival (LD50-180) and histopathology. Detailed histological examination will be dealt with elsewhere. Chemicals/CAS: cyclophosphamide, 50-18-0; cytarabine, 147-94-4, 69-74-9; Cyclophosphamide, 50-18-0; Cytarabine, 147-94-4