Rearrangement of the beta and gamma chain genes of the TCR gene complex and of the Ig heavy chain genes were examined in three cases of childhood acute mixed lineage leukaemia. Blast cells, classified morphologically as acute lymphoblastic leukaemia (ALL) in one child and acute non-lymphocytic leukaemia (ANLL) in the other two, all co-expressed markers associated with both T (CD7, TdT) and myeloid (CD33) cells. Cytogenetic analysis detected abnormalities associated with myeloid leukaemia. Immunoglobulin heavy chain genes were not rearranged in two patients but a novel rearrangement was seen in the third. No rearrangement of the beta or gamma chains of the T-cell receptor complex were seen. Acute mixed lineage leukaemia may thus arise from a pluripotent precursor cell capable of both lymphoid and myeloid differentiation.