We have raised a monkey antiserum that is selectively reactive with human leukemic myeloblasts by immunization with a glycoprotein antigen (AMLSGA) released from myeloblasts in short-term culture. Antimyeloblast activity can be demonstrated using complement-dependent cytotoxicity or indirect immunofluorescence. Selective antimyeloblast activity is retained following absorption with leukemic lymphoblasts or lymphocytes, nonleukemic lymphocytes, neutrophils, or mononuclear cells from nonleukemic bone marrow. Anti-AMLSGA antisera are not reactive with B-cell-enriched cell populations, and antaimyeloblast activity is not reduced by absorption with Ia-positive cells. Anti-AMLSGA is a useful reagent for identification of human leukemic myeloblasts.