To estimate intracellular interleukin-1 receptor antagonist (icIL-1ra) expression in the bone marrow cells from adult patients with chronic myelogenous leukemia (CML), flow cytometry (FCM) assay was used for detecting the mean icIL-1ra fluorescence intensity per cell (equivalent to it's expression level) in different groups of cells from normal and CML patient bone marrows by 15 monoclonal antibodies with different fluorescent marker. The results showed that all of marrow nucleated cells express IL-1ra, but its expression level in granulocytes was the highest and that in lymphocytes was the lowest. The icIL-1ra expression level was significantly lower in nucleated marrow cells, granulocytes and lymphocytes from the marrow of 17 untreated CML patients than that from the marrow of 8 normal. The mean icIL-1ra fluorescence intensity was significantly lower in marrow nucleated cells, granulocytes and lymphocytes in 13 CML patients with marrow blasts >or= 10% than that in 43 CML patients with marrow blasts < 5% or than that in 9 CML patients with marrow blasts 5% - 10%. It was concluded that the lower expression of icIL-1ra in CML marrow nucleated cells might be involved in the evolution of CML.