Early studies of terminal transferase (TdT) expression in acute leukemia indicated that this enzyme is only found in acute leukemia of lymphoblastic type (ALL). More recently, however, several reports have suggested that TdT-positive blast cells may be found in a substantial number of cases of acute myeloid leukemia (AML). In this study, a sensitive immunoalkaline phosphatase procedure (the APAAP technic) has been used to stain normal and neoplastic blood and bone marrow samples for TdT with the use of both polyclonal and monoclonal anti-TdT antibodies. As expected, most cases of ALL studied (63 of 65) were TdT-positive. In addition, however, blast cells in 22 out of 59 (37%) cases of AML were stained by anti-TdT antibodies. Both nuclear and cytoplasmic localization were seen in each type of acute leukemia. These findings, together with previous immunocytochemical and biochemical studies, suggest that a substantial number of cases of AML express TdT (usually in a minority of blast cells) and that the frequency with which these cases are detected is directly related to the sensitivity of the staining technic used.