A monoclonal antibody termed NC-1 was produced which binds to an antigen present on the human promyelocytic leukemia cell line HL-60 and peripheral blood neutrophils. The antibody bound to the majority of promyelocytes in the HL-60 culture, but not to myeloblasts. No antibody reactivity was detected to a range of other cell lines or to a limited number of normal human tissues. Peripheral blood lymphocytes, monocytes, basophils, eosinophils, erythrocytes and platelets did not react with the antibody. Bone marrow smears exhibited binding of NC-1 to myeloid cells at the promyelocyte and later stages of differentiation along the granulocyte lineage. The KG-1 myeloblastoid and U937 myelomonocytic lines could be induced to express the antigen, when exposed to neuraminidase which removes terminal sialic acid from carbohydrate residues. Similarly myeloblasts in bone marrow samples bound NC-1 when they were treated with neuraminidase. HL-60 cells induced to differentiate to granulocytes by treatment with retinoic acid continued to express the antigen. A similar result was observed when HL-60 cells were induced to differentiate to monocytes, even though blood monocytes failed to bind the antibody. These data indicate that the antibody NC-1 reacts with an antigen expressed on myeloid cells beyond the promyelocyte stage of differentiation.