Normal human peritoneal cells collected during elective laparatomy from patients with gallbladder stones without clinically detectable inflammatory changes were characterized phenotypically with immunocytochemical method and flow cytometry, with special attention paid to the presence of memory cells. The responsiveness of normal PCs to mitogen and, specifically, the role of peritoneal macrophages in this process was studied. The peritoneal cells consisted of 45% of monocytes/ macrophages (CD68+), as many as CD2+ T lymphocytes, 8% CD57+ NK and K 2% CD22+ B, cells. The CD4/CD8 ratio was 0.4. The peritoneal cells did not express interleukin-2 (CD25+) and transferrin receptors (CD71+) on their surface. Approximately 49% of the peritoneal cells were class II MHC antigen positive cells. Two per cent of S100+ dendritic cells were found. Flow cytometric two-colour analysis revealed that the majority of peritoneal CD4+ (92.4%) and CD8+ (73.1%) lymphocytes, while only 50.2% of CD4+ and 30.1% CD8+ peripheral blood cells expressed simultaneously the CD45R0 (UCHL1) molecule, which is characteristic to the memory/effector T-cell subpopulation. Peritoneal T lymphocytes responded to the mitogens less than peripheral blood lymphocytes of the same individual. Supplementation of cell culture with anti-macrophage (anti-CD68) and anti-HLA-DR MoAb brought about a dose-dependent decrease of proliferative peritoneal cell response to ConA. The authors conclude that human peritoneal cell population comprises a high proportion of T lymphocytes and macrophages capable of presenting antigens to peritoneal lymphocytes. High prevalence of memory lymphocytes points to the preparedness of these cells to react with invading antigens most likely of gut bacterial origin.