Between January 1980 and May 1981, 1966 marrow or blood samples from leukaemia patients were tested for terminal deoxynucleotidyl transferase (TdT) using nuclear immunofluorescence. The cells were also tested with a panel of immunological markers including monoclonal antibodies. Of 869 TdT positive cases detected, 555 were diagnosed as ALL and 32 as blast crisis of CGL; 226 were provisionally diagnosed as 'acute leukaemia' and finally diagnosed as ALL partly on the basis of immunological data; 56 TdT+ cases were provisionally diagnosed as acute non-lymphocytic or myeloid leukaemia; 266 cases of AML and 177 cases of CGL in blast crisis were TdT negative. Eleven of the above 'AML' cases were anti-cALL+ as well as TdT+ and were re-diagnosed and treated successfully as cALL. The remaining 45 were anti-cALL negative and finally diagnosed and treated, at least initially, as AML. Eleven of these cases had only 5-10% TdT+ cells which could have been normal, non-myeloid cells. Twenty cases had 11-50% TdT+ cells and 14 cases had 50-100% TdT+ cells. Of these latter two groups, details on 28 patients were available for evaluation. Three cases on review had no definitive myeloid cytochemistry and were haematologically AUL with a null-ALL phenotype (TdT+ DR+ cALL-). In 14 cases there was a large overlap (greater than 75%) of the proportion of cells with myeloid cytochemistry (Sudan black, peroxidase or esterases) and TdT; individual blast cells were therefore expressing these markers concurrently. In the remaining cases, mixtures of TdT negative myeloid and TdT+ (lymphoid?) cells may have coexisted although this was not proven unequivocally. Twenty-two cases of newly diagnosed TdT+ 'AML' received induction chemotherapy for AML (DAT regime) and only six (37%) obtained a complete remission. It is concluded that TdT positive 'myeloid' leukaemias do occur, albeit infrequently (approx. 5%) and may have a relatively poor prognosis.