A double immunofluorescence technique, using antibodies to terminal transferase (TdT) and a 165-kilodalton myeloid differentiation antigen (p165), has been used to investigate the phenomenon of TdT expression in cases of acute myeloid leukaemia (AML). Five cases of AML were shown to have significant (18-90%) numbers of leukaemic cells that concurrently expressed both TdT and p165 myeloid surface antigen. Examination of nonleukaemic bone marrow cells showed that the vast majority of normal TdT+ cells are p165 negative. However, in 5 of the 11 samples analyzed, rare cells staining for both p165 and TdT were found. These results suggest that some cases of TdT+ AML may arise from the clonal expansion of rare "biphenotypic" precursor cells existing in normal bone marrow.