Mononuclear leucocytes (mainly lymphocytes), isolated by Ficoll-Paque gradient centrifugation of blood freshly drawn from a haemophilia A patient with chronic non-A, non-B hepatitis (NANB), caused NANB when infused in a susceptible chimpanzee. Plasma from the same patient also transmitted the disease, as witnessed by elevated levels of liver enzymes in serum, histopathological signs of viral hepatitis and submicroscopic cytoplasmic alterations in the hepatocytes, considered to be specific for NANB in chimpanzees. In contrast, neither lymphocytes nor plasma from a chimpanzee apparently fully recovered from two episodes of NANB had the same effect.