An epidemic outbreak of non-A, non-B hepatitis occurred in 1977/78 involving 30 donors at a plasmapheresis center. A chimpanzee inoculated with serum of one donor developed non-A, non-B hepatitis with characteristic tubular alterations in the endoplasmatic reticulum. Infections were detected over a period of several months, with two well defined peaks in December 1977 and between the end of January and the beginning of February 1978. Epidemiological data suggested a spread within the center. The most probable mode of transmission was contamination with serum from plastic bags used for reinfusing erythrocytes. The estimated mean incubation time was 41 days (range 27 to 59 days).