A membrane-coated virus having a diameter of 85-90 nm and containing a 40-45 nm core was found to replicate in cell cultures derived from chimpanzee liver after inoculation of serum containing infective non-A, non-B (NANB) hepatitis viruses from two independent sources. Replication of this agent was not observed when the same cells were inoculated with a chloroform-extracted inoculum or were left uninoculated. Replication involves assembly of virus cores on tubular structures similar to those seen in liver cells of chimpanzees infected with most isolates of NANB virus.