Autoantibodies of the IgG isotype, specifically directed against intestinal alkaline phosphatase (IAP), occur transiently in the majority of sera from patients with acute bacterial infections. Sometimes they are observed in autoimmune diseases. Using a T cell proliferation assay, it was found that isolated peripheral blood mononuclear cells (PBMC) from IAP autoantibody (IAPA)-positive patients (n = 18) responded significantly to IAP, whereas proliferation could not be induced in PBMC from healthy donors (n = 11). Significant stimulation of PBMC from patients (n = 11) was not obtained by use of transferrin, a common autoantigen in humans, indicating the specificity of stimulation of IAP-reactive T cells. Furthermore, T cell proliferation was observed when a highly purified IAP fragment (CNBr 21) spanning amino acids 334-462 of the primary structure of IAP was used as antigen. Thus, it was shown that an immunodominant T cell epitope resides within the CNBr 21 fragment which also contains a discontinuous B cell epitope as evaluated previously. Double immunocytochemical staining of T cell-depleted PBMC with IAP and an anti-human CD5 antibody allowed the detection of CD5+ B lymphocytes, which probably produce natural IAPA (nIAPA). These nIAPA-specific CD5+ B cells occurred with approximately the same frequency among T cell-depleted PBMC from healthy donors and those from patients. In contrast, IAPA-producing CD5- B cells were found in B cell-enriched preparations from patients, but not in those from healthy individuals.