Multiparameter analyses have been carried out with recently developed enzyme and membrane markers in 300 patients with various leukaemias including ALL, AML, but excluding Ph1 positive leukaemias. TdT enzyme levels were particularly valuable in the differential diagnosis of adult acute lymphoid and myeloid leukaemias. The levels were raised in 108 (94%) of the 115 patients who were considered to be non-T, non-B ALL on membrane marker and morphological analysis; all seven cases giving negative TdT results in this group were young children. Unexpectedly high levels were seen only in three (4.1%) of 73 cases of acute myeloid leukaemia verified by histochemistry and membrane markers. Anti-ALL serum was a most useful reagent in childhood leukaemias but blasts from 19 patients (10% of childhood ALL cases and 29% of adult ALL cases) failed to react with the serum in spite of TdT positivity. Strongly ALL+ blasts were seen only in non-T, non-B ALL and some undifferentiated leukaemias. Weakly ALL+ blasts were seen in seven of 32 cases of thymic ALL (Thy-ALL) but in other respects these blasts expressed Thy-ALL features, such as strong reactivity with anti-T cell (HuTLA) serum, negativity with anti-Ia-like serum and raised TdT. The combination of tests was particularly useful in 32 cases of undifferentiated leukaemia: in 10 of these cases TdT positivity indicated the probable 'lymphoblast', nature of blast cells: the remaining 22 cases remained unclassifiable with the markers used. The analysis revealed other interesting variant forms of leukaemias.