Background/Aims: In chronic active liver diseases (CALD) with viral aetiology, a population of plasma cells localised in the piecemeal necrosis areas was previously detected by means of autoradiography after in vitro3H-proline incorporation, a method which proved much more sensitive than conventional immunohistochemical procedures. These plasma cells, characteristically located in niches among hepatocytes, in close contact with collagen fibrils, have been hypothesised to exert a role in fibrogenesis stimulation, and particularly in collagen synthesis, possibly through secretion of lymphokines. Specifically, we investigated the presence of interleukin-1, well known to play a crucial role in inflammation and production of collagen by epithelial cells, and to be present in activated plasma cells of myeloma. Methods: The immunohistochemical localisation of interleukin-l/beta in biopsies of patients suffering from chronic active hepatitis was studied, using an affinity purified rabbit polyclonal antibody. Results: The strongest interleukin-l/i immunostaining was observed in the above-described plasma cell population, identified by anti-immunoglobulin antibodies, and 3H-proline incorporation. Conclusions: The ability of plasma cells to produce interleukin-1 during viral CALD suggests that in these pathologies plasma cells play a major role, mainly of paracrine nature. Interleukiu-1, possibly together with other mediators, might in turn stimulate the production of collagen. Hepatocytes of the piecemeal necrosis area appear to be possible candidates for this synthesis, as they show a significant labelling after 3H-proline incorporation, which is absent from hepatocytes far from necrotic areas.