Seven populations of human leukaemic cells were implanted i.v. into sublethally irradiated severe combined immunodeficient (scid) mice. Growth of leukaemia was monitored by labelling murine peripheral blood (PB) cells with an anti-HLA monoclonal antibody and flow cytometric analysis. Two of the populations transplanted were fresh acute lymphoblastic leukaemia (ALL) bone marrow (BM) cells which both caused sustained proliferative growth in scid mice. Human cells accounted for up to a mean of 87% of the total nucleated cells (TNC) in the PB of these mice between weeks 12-15. One of these populations was passaged into fresh mice and frank leukaemia was again established. Three populations of cryopreserved acute myeloblastic leukaemia (AML) cells (2 obtained from PB and 1 from BM) and one population of cryopreserved biphenotypic acute leukaemia BM cells, only grew to a maximum of 4% within the 15 week period of the experiment. A cell population from an AML cell line (HL60), however, did engraft and proliferate resulting in a rapid deterioration of these mice between weeks 3-6 when the proportion of human cells accounted for 9% of the TNC in the PB.