Organ homogenates from kits that died of interstitial pneumonia were inoculated into adult Aleutian disease virus (ADV)-negative mink and shown to contain infectious ADV. Acute interstitial pneumonia was experimentally reproduced with the organ homogenate but only by inoculation of newborn kits born from ADV-negative dams. Older kits and kits from ADV-positive dams did not develop interstitial pneumonia, but later developed the classic form of Aleutian disease. Electron microscopic examination was done on purified suspensions of defined ADV isolates and on purified organ homogenates from kits with spontaneous or experimental interstitial pneumonia. In kits from both groups a virus, morphologically resembling the defined ADV isolates, was demonstrated. Findings of intranuclear inclusion bodies and intranuclear ADV antigen in alveolar type-II cells in affected lungs and the lack of immunologically mediated lesions suggest that lung lesions result from primary viral injury to alveolar type-II cells. Experiments also showed that infection of dams with ADV before pregnancy decreased the number of kits per mated dam and infection with ADV in mid-pregnancy caused fetal death, fetal resorption, or abortion.