An indirect immunofluorescent technic was used to identify cells with terminal deoxynucleotidyl transferase (TdT) activity in bone marrow smears from 169 patients without a clinically or morphologically apparent hematologic malignancy: 73 normal individuals, 27 bone marrow transplant recipients, 25 with neuroblastoma or retinoblastoma, 19 with acute lymphoblastic leukemia (ALL) in remission, and 25 with miscellaneous benign conditions. In selected cases, additional bone marrow smears were investigated for TdT-positive cells, using one of two immunoperoxidase methods, the peroxidase-antiperoxidase (PAP) technic or the avidin-biotin-peroxidase complex (ABC) technic. Superior preparations were obtained using the ABC technic. Fifty-eight of the 169 smears contained 2% or more TdT-positive cells, range 2%-20%. The morphologic characteristics of the TdT-positive cells in phase-contrast illumination and in Wright's-Giemsa counterstained immunoperoxidase preparations indicated that many of these cells may be hematogones. TdT-positive cells in the bone marrow of a patient with ALL in remission following chemotherapy or bone marrow transplantation do not necessarily denote relapse.