Severe necrotising hepatitis, with microscopic brain lesions containing multi-nucleated giant cells, was observed in specified-pathogen-free nu-nu mice, contained in a semi-protected environment, but within the same room as other, conventional, mice. An entire group of 40 nu-nu mice died within a month, with no evidence of bacterial or parasitic infection. However, cell-free liver filtrates reproduced the liver lesions in specified-pathogen-free Swiss-Schneider mice injected intraperitoneally at up to 21 days of age, but not in baby rats and hamsters.