Skin reactivity to HBsAg was studied in patients with type B acute and chronic hepatitis and in healthy controls. HBsAg preparations containing 50 micrograms/ml of antigen both with and without alum elicited positive skin reactions in all seven anti-HBs+ persons. Histological examination of skin tissue from the reactive area using monoclonal antibodies to the cell surface of lymphocytes revealed accumulation of Leu-3a positive lymphocytes, showing that the inflammation was a typical delayed type hypersensitivity (DTH) reaction. Irrespective of the presence of HBeAg and anti-HBe, patients with chronic type B hepatitis responded clearly to PPD and SK/SD antigens, whereas they did not exhibit DTH skin reactivity to HBsAg. In contrast, although patients with acute type B hepatitis did not exhibit specific DTH a reaction to HBsAg in the acute period, they began to develop DTH skin reactivity to HBsAg in the convalescent phase of the disease preceding the appearance of anti-HBs antibody. It is suggested that DTH reaction to HBsAg might play an important role in the pathogenesis of type B viral hepatitis.