Nitric oxide (NO) is a biological mediator which is synthesized from L-arginine by a family of nitric oxide synthases (NOS). We have studied the expression of the inducible NOS (iNOS) by bone marrow cells from the patients with myelodysplastic syndromes (MDS) at the mRNA level by RT-PCR assay and at the protein level by immunohistochemical staining using a specific anti-iNOS monoclonal antibody. The iNOS message was present in 92% of bone marrow tissues from MDS patients (11 out of 12) by an examination using RT-PCR. Basically, iNOS message was negative or very weak in control (1/9) and AML (0/7) cases. This was supported by immunohistochemical findings that the iNOS was positive in most of the bone marrow samples from MDS patients (9 out of 12), while bone marrow cells of control (O out of 12) and AML (O out of 5) cases were basically negative. Double immunostaining for CD68 antigen, which is a marker for macrophage lineage cells, and iNOS was performed on MDS bone marrow sections. iNOS was dominantly localized to bone marrow macrophages, although a part of myeloid cells were also positively stained with anti-iNOS antibody in a part of cases. These results indicated that there is some in vivo induction of iNOS expression for local NO production that might be involved in the dysregulation of hematopoiesis in bone marrow of MDS.