24 patients with myelodysplastic syndrome (MDS) were examined for the following items. T lymphocyte subsets of five subtypes of MDS examined with monoclonal antibody were less than that in normal controls. After incubation of peripheral blood lymphocytes with PHA at 37 degrees C with 5% carbon dioxide, the amount of HLA-DR positive cells and NKH1 positive cells were less than normal, NK cell activity was measured with cytotoxicity assay using 51 Cr labeled target K562 cells, the results showed that NK activity in MDS was less than that in controls. As mentioned above, the results indicated that the activity and quantity of T lymphocytes and NK cells in MDS were abnormal.