ABSTRACT—1) A NANB agent of human origin was inoculated in HBV‐immunized chimpanzees. Infection was proven in two animals and serially passed to two others. The absence of anti‐HBc in serum and the absence of HBsAg and HbcAg in liver are arguments against the HBV nature of the transmitted infection. Moreover, the reproducible appearance of the NANBcAg/Ab system at each passage from man to chimpanzee and from chimpanzee to chimpanzee, a response not elicited in control animals, suggests that this reaction may be a specific immunologic marker for the strain. 2) NANB infection was transmitted in all chimpanzees. 3) Distinctive hepatic morphologic features were obtained in the liver biopsies of the human donor and the inoculated chimpanzees: eosinophilic alterations of hepatocytes and numerous inflammatory cells. Inflammation was more prominent than necrosis, appearing earlier and lasting longer, but was not topographically close to the eosinophilic changes. 4) On electron microscopy, particles characteristic of NANB agent were observed in the cytoplasm of the hepatocytes. No particles were demonstrated in the nucleus of these