1. A case of chronic myeloid leukaemia is described in which the majority of dividing cells in a short-term leucocyte culture had 45 chromosomes: a small acrocentric was missing. The Ph1 chromosome appeared to be present in these cells. 2. Other cells had 46 chromosomes and were apparently normal. The chromosomes of these cells were distinguishable from those of the presumed leukaemic cells by virtue of their narrower and more intensely-staining chromatids. 3. The nature of the chromosomal changes in this case are briefly discussed in relation to those described in other cases of chronic myeloid leukaemia.