We have investigated two chronic myeloid leukaemic (CML) patients with a stable Ph1 mosaicism to see whether the Ph1 positive cells belonged to more than one lineage. Individual erythroid and granulocytic colonies were cytogenetically examined. Ph1-positive and Ph1-negative colonies were found both in the erythroid and granulocytic lineage supporting the contention that CML with a stable Ph1 mosaicism as well as classical CML involves a pluripotent stem cell.