Studies with glucose-6-phosphate dehydrogenase (G6PD) isoenzymes have demonstrated that chronic myelogenous leukaemia (CML) is a clonal disorder of pluripotent haematopoietic stem cells which are capable of differentiation to myeloid cells, monocytes, erthrocytes and platelets. It has been observed recently in G6PD heterozygous patients with chronic phase CML that the non-E-rosetting lymphocytes were restricted to a single enzyme type, indicating that some lymphoid cells must also arise from the leukaemic clone. Surface or cytoplasmic immunoglobulin could be detected in up to 46% of the cells of these isolated non-T-lymphocyte populations, which suggested that cells from the CML clone were capable of differentiating into B lymphocytes. To investigate this further, we established Epstein-Barr virus (EBV)-transformed B-lymphoblastoid cell lines derived from patients with CML and studied chromosomes and G6PD to determine whether progenitor B lymphocytes for any of the cell lines had originated from the CML clone. We report here direct evidence that immunoglobulin-synthesizing B lymphocytes can arise from the CML stem cell clone.