Two from a group of approximately 50 C.B-17 scid-bg mice were examined because of lethargy, dehydration, and rough coat. Three months prior to development of clinical signs of disease, mice of this study had been surgically implanted with fetal bovine liver, thymus, and lymph node. At necropsy, marked splenomegaly and mild hepatomegaly were observed in both animals. Large areas of necrosis and inflammation, with associated intracytoplasmic granular basophilic inclusions, were observed in histologic sections of multiple organs. Aerobic and anaerobic culturing of the liver yielded negative results. Six months after the initial case, four more reconstituted scid-bg mice from a different fetal donor had identical clinical, gross, and histologic signs of disease. To determine whether the basophilic inclusions represented an infective agent, 4-month-old immune-naive C.B-17 scid-bg mice were inoculated intraperitoneally with a liver and spleen homogenate from an affected mouse. Two weeks after inoculation, mice developed clinical signs of disease and lesions identical to those seen in the signal mice. On ultrastructural examination of the liver, pleomorphic bacteria were found in large cytoplasmic vacuoles of hepatocytes. Bacterial DNA was amplified from the liver, using primers that amplify a segment of the 16S rRNA gene from many bacterial species. Sequencing of the polymerase chain reaction (PCR) product revealed gene sequence identical to that of Coxiella burnetii, the agent of Q-fever. These results highlight the need to consider infective agents of the donor species when working with xenografted animals.