Scid mice develop a severe, chronic, and lethal IBD 3-6 months after engraftment of gut wall from immunocompetent congenic donors, induced by donor-derived CD4+ T cells migrating from the graft. We have investigated intracellular T-helper type 1 (Th1) cytokines in the spleens of gut wall-transplanted scid mice with IBD. Increased fractions of interferon-gamma (IFN-gamma), tumour necrosis factor-alpha (TNF-alpha) and IL-2-positive CD4+ T cells were found in the spleens of diseased mice compared with control mice. Moreover, a small but significant population of CD4+ T cells which stained positive for granulocyte-macrophage colony-stimulating factor (GM-CSF) was found in scid mice with IBD but was virtually absent in congenic non-scid control mice. Cloning of granulocyte/ macrophage colony-forming cells (G/M-CFC) revealed that both non-transplanted scid mice and scid mice with IBD had an 8-14-fold increase in splenic G/M-CFC compared with control mice. No significant difference in the number of G/M-CFC per total spleen was found between non-transplanted and disease scid mice, although both groups of mice showed a nearly two-fold increase compared with control mice. G/M-CFC were never found in the thymus, liver or lymph nodes of diseased mice. Immunohistochemistry revealed that the multinucleated giant cells observed in the gut wall of diseased mice did not represent haematopoietic foci, but were derived from macrophages. These observations point towards a dominant role for Th1-type CD4+ T cells in the immunopathogenesis of IBD, whereas haematopoiesis does not seem to be affected by the development of the disease.