Eight patients, convalescent from acute hepatitis, all negative for HBsAg, anti-HBc, anti-HA-IgM, anti-CMV and anti-EB, and all positive for non-A, non-B related antigen and/or antibody in at least one sample during the course of disease, were investigated. Their specific cell-mediated immunity was tested using the leukocyte migration inhibition test in presence of a previously described non-A, non-B related antigen. A control group consisted of ten individuals (two normal subjects, six patients recovering from acute hepatitis B and two with autoimmune chronic active hepatitis). Leukocyte migration was inhibited by the antigen in seven of the eight patients. The test was negative in all control subjects. It is thus demonstrated that after recovery, patients with non-A, non-B viral hepatitis develop a cellular immune response specific for purified non-A, non-B related antigen. The association between the described antigen and viral hepatitis non-A, non-B can now be established not only by immunoprecipitation, but also by a cellular test.