26 patients with viral hepatitis were selected on the basis of negative responses to serological tests for hepatitis B virus and hepatitis A virus infections. The illness was acute in 19 patients and chronic persistent in the remaining 7. Immunodiffusion tests showed a single antigen-antibody system in the sera of all 26 patients. Each patient had at least one serum sample positive for antigen or antibody. In 1 post-transfusion case, the corresponding immunological marker was found in the donor. This system was considered specific for the disease, since the markers could not be detected in sera from patients with other liver conditions or immune complex diseases, or in sera from healthy blood donors without a history of hepatitis. The antigen could be separated from contaminating serum proteins. The purified preparation showed immunological identity with all our antigen-containing sera and with samples of a study already published. These results suggest the existence of an antigen associated with non-A, non-B, viral hepatitis and probably related to an infectious agent responsible for all the cases in this study.