It has long been suggested that the blastic transformation in some patients with Ph1-positive chronic myelocytic leukaemia (CML) may be lymphoid in nature. It has recently been postulated that some patients with CML may undergo a T lymphoblastic crisis because the leukaemic blasts from these patients have high terminal deoxynucleotidyl transferase (TdT) activity and that some patients may undergo a non-T/non-B lymphoblastic crisis since leukaemic blasts from a majority of morphologically lymphoid type CML-BC cases react with antiserum specific for non-T/non-B acute lymphoblastic leukaemia (ALL). The present study shows that leukaemic blasts from each of six patients with Ph1-positive chronic myelocytic leukaemia-blastic crisis (CML-BC) exerted a strong stimulation on allogeneic lymphocytes in 'one-way' mixed lymphocyte reaction. There was no apparent difference in stimulating capacity between morphologically myeloid type (four cases) and lymphoid type (two cases). The stimulating capacity of leukaemic blasts from patients with CML-BC was quite similar to that of blasts from all patients with acute myeloblastic leukaemia (AML) and from some patients with non-T/non-B type ALL. Leukaemic blasts from a patient with T-cell type ALL and cultured leukaemic T lymphoblastoid cells (2 lines) consistently failed to stimulate while cultured leukaemic null-cells (4 lines) consistently exerted a strong stimulation in 'one-way' mixed lymphocyte reaction. These observations suggest that leukaemic cells from patients with CML-BC, morphologically lymphoblastic type, are not T lymphoblasts although the possibility that these cells are non-T/non-B lymphoblasts cannot be ruled out entirely.