Expression of I and sialosyl-I antigens was examined using specific monoclonal antibodies. The anti-I antibody C6 reacted with monocytes (24%), T cells (55%), B cells (80%) but not with neutrophils (4%), bone marrow (BM) CD34+ cells (2%) or mobilized peripheral blood (PB) CD34+ cells (1%). The anti-sialosyl-I antibody NUH2 reacted with monocytes (38%) and BM CD34+ cells (41%) but not with T cells (2%), B cells (0%) or neutrophils (1%) and it hardly reacted with mobilized PB CD34+ cells (8%). Flow cytometric analyses of CD34+ cells enriched from BM showed that most of the sialosyl-I cells expressed CD13, CD33, CD117, and HLA-DR. Sialosyl-I+ CD34+ cells isolated from BM produced a large number of granulocyte-macrophage colonies and macrophage colonies. Therefore, sialosyl-I+ CD34+ cells are suggested to be colony-forming units granulocyte-macrophage (CFU-GM) and colony-forming units macrophage (CFU-M). BM CD34+ cells cultured in medium containing cytokines produced I+ CD14+ monoblasts and sialosyl-I+ CD14+ monoblasts. Leukemic cells from patients with acute myeloid leukemia were I-negative (32/32) and sialosyl-I-positive (one/32). Leukemic cells from patients with acute lymphoid leukemia were I-positive (four/ten) and sialosyl-I-negative (ten/ten). These results indicate that (1) the I antigen is broadly expressed by monoblasts, monocytes, lymphocytes, and leukemic lymphoblasts, and (2) the sialosyl-I antigen is expressed along the normal differentiation of CFU-GM to monocytes.