Five patients with clinical and laboratory features typical for juvenile chronic myeloid leukaemia (JCML) are presented. Rearrangement of the j joining region of the immunoglobulin heavy chain (Jh) was demonstrated in three children out of five analysed. As no Vh to DhJh nor kappa light chain rearrangements were demonstrated, it is reasonable to speculate that the transforming event of the stem cell happened at the stage when Dh to Jh rearrangement took place. As the monocytic lineage is prominent in JCML, it is suggested that the transforming event happens in a unique stem cell with intermediate differentiation towards the myelomonocytic as well as the B-lymphatic lineage. This stem cell, which is present at a certain stage of embryogenesis, disappears later. Such an early `hybrid' cell is sometimes involved in leukaemias of early infancy, and may be the transformed cell in some cases of infantile leukaemia.