Intraportal lymphoid nodules have been observed in liver biopsy specimens from patients with autoimmune liver disease and chronic active hepatitis C. They are characterized by a nodular lymphocyte aggregate with a germinal center. The purpose of this in situ immunophenotyping study was to determine immunohistochemically the phenotype of immunocompetent cells in and around intraportal lymphoid nodules in patients with chronic hepatitis C. For comparison, we also examined lymphoid nodules in patients with autoimmune liver disease. Liver biopsy specimens from 13 unselected patients with CAH and hepatitis C antibodies and from 7 unselected patients with autoimmune liver disease seen during the same period were studied. Sections of snap-frozen specimens were immunostained with monoclonal antibodies directed against different subsets of T cells, B cells, follicular dendritic cells and macrophages. Intraportal lymphoid nodules were observed in 11 of 13 patients with chronic hepatitis C. They appeared as lymphoid follicles with activated B cells in germinal centers surrounded by a follicular dendritic cell network. A mantle zone of B cells was present around the aggregate of activated B cells. A T-cell zone comprising CD4-positive helper T cells, CD8-positive suppressor/cytotoxic T cells and a few CD25-positive activated T cells expressing interleukin-2 receptor was seen at the periphery of the nodules. Human leukocyte antigen DR was expressed by B cells and by T cells, indicating that T cells were activated. The immunocompetent cells observed in piecemeal necrosis were predominantly CD4 helper T cells, whereas those seen in the lobule were predominantly CD8 suppressor/cytotoxic T cells.(ABSTRACT TRUNCATED AT 250 WORDS)