Monoclonal antibody T-11, considered specific for the sheep erythrocyte rosette-associated antigen of T cells, reacted with leukemic blasts from four of 23 patients with morphologic and cytochemical criteria for acute myeloblastic leukemia (AML). Although 83%, 87%, 88%, and 96% of the blasts from these patients reacted with T-11, only one patient demonstrated a small percentage of heat-stable E rosettes (5%). Antibody 9.6, which also reacts with the E rosette-associated antigen, was tested on blasts from two of the T-11-positive patients and was also strongly reactive (96% and 98%). Dual staining of blasts from these two patients demonstrated a small number of cells that simultaneously expressed the E rosette-associated antigen and myeloid- associated cytochemistries (myeloperoxidase [MPO] and Sudan black B). Additionally, leukemic blasts were identified that simultaneously expressed the E rosette-associated antigen and contained Auer rods. Antibody OKT-11 immunoprecipitated a 48,100-dalton glycoprotein from these leukemic blasts that is similar in molecular weight to that previously determined for the T cell surface protein (Tp50), thus providing strong evidence that this molecule can be found in some cases of AML. Because cells simultaneously expressing both the E rosette- associated antigen and MPO were identified, it would appear likely that leukemic blasts with only the E rosette-associated antigen or only MPO arose from the same progenitor. Our findings further demonstrate that the epitopes identified by antibodies OKT-11, T-11, and 9.6 are not always associated with, or sufficient for, 37 degrees C E rosette formation and can be found on blasts from patients with AML.