Toga virus-like particles (typically 60-70 nm: enveloped with small surface spikes) were detected in the native hepatectomy specimens in 7 of 18 patients grafted for acute liver failure attributed to sporadic non-A, non-B hepatitis and in 2 patients grafted for fulminant hepatitis attributed to anti-epileptic drug hepatotoxicity. These particles were not detected in the hepatectomies from 12 other patients grafted for other causes of acute liver failure, 12 for various chronic liver diseases, and 2 histologically normal livers. Acute hepatic failure, characterized histologically by severe haemorrhagic necrosis, developed 7 days after grafting in 5 patients, all in the non-A, non-B group with toga virus-like particles in native liver. Similar virus-like particles were detected in all grafts and were in greater abundance than in the native livers. The agent may be novel because pre- and post-grafting sera were negative for antibodies against representative panels of arboviruses and in first and second generation antibody tests for hepatitis C virus.