Lymphocyte subpopulations were measured in the blood of 17 patients with megaloblastic anaemia due to vitamin B‐12 deficiency. 14 patients had pernicious anaemia and 3 others were gastrectomized. By using monoclonal antibodies recognizing T cell surface markers and immunofluorescence microscopy, we found a significant decrease in the number of circulating suppressor T cells and an increase in the ratio of helper to suppressor T lymphocytes in pernicious anaemia patients. This finding may be related to other immune abnormalities found in pernicious anaemia, e.g. the presence of multiple autoantibodie