An antigen-antibody system has been identified by immunofluorescence in patients with non-A, non-B hepatitis. The non-A, non-B antigen was localized in the hepatocyte nuclei of liver biopsies from patients with acute post-transfusion or sporadic non-A, non-B hepatitis and in those from patients with chronic post-transfusion non-A, non-B hepatitis, the percentage of positive cells being most prominent in patients receiving immunosuppressive treatment. Absence of the antigen in normal livers and in livers from patients with type B hepatitis infection indicated its specific association with non-A, non-B infection. Antibody reacting with the nuclear antigen became detectable in serum during post-transfusion acute non-A, non-B hepatitis in 11 out of 15 cases; it was absent before transfusion. Six out of 12 cases of sporadic acute non-A, non-B hepatitis were also found to produce the antibody, which was repeatedly found to be absent during the acute phase in five patients with type A and in eight with type B hepatitis. The non-A, non-B antibody, mainly an IgM antibody, persisted in serum for prolonged periods of time after onset, both in patients showing biochemical resolution of their illness and in those who continued to have liver damage after the acute phase. Accordingly, eight out of nine patients with chronic non-A, non-B hepatitis were found positive for the antibody in serum, seven at the time the non-A, non-B antigen was detected in their liver. Thus this non-A, non-B associated antigen-antibody system shares remarkable similarities of behaviour with the "core" system of the hepatitis B virus.