The portal and acinar zone 1 (periportal) inflammatory infiltrate in acute hepatitis A (HA) strongly resembles piecemeal necrosis (PMN) in chronic hepatitis B (HB). The latter infiltrate has been characterized in detail previously, and a predominance of suppressor/cytotoxic T cells has been demonstrated. For comparison we analyzed the infiltrate in acute HA in three liver biopsies, applying a broad panel of monospecific and monoclonal antibodies. In two cases, helper/inducer T cells surrounded by B lymphoid cells represented the main lymphoid subset in all portal areas. In the third liver biopsy, a case of acute HA superimposed on a pre-existing chronic HB, the cellular composition of the inflammatory infiltrate varied from one portal tract to another. Despite the morphological resemblance between the inflammatory infiltrate in acute HA and PMN in chronic HB, a clear-cut difference in the composition of both infiltrates is obvious from our results. By virtue of the presence of numerous suppressor/cytotoxic T cells, PMN in chronic HB represents the morphological expression of a cellular immune response, whereas the predominance of helper/inducer T cells, admixed with B cells and plasma cells in acute HA represents the morphological expression of a humoral immune response.