When marrow cells from Philadelphia chromosome (Ph) positive chronic myelogenous leukemia (CML) patients are placed into long-term culture, Ph-positive progenitor numbers typically decline rapidly. Karyotypically normal cells derived from persisting, but previously undetectable, precursors can then frequently be demonstrated. To investigate the origin of these cytogenetically normal progenitors, long-term cultures were established with marrow from a girl with CML who was also heterozygous for the glucose-6-phosphate dehydrogenase (G6PD) enzyme variants A and B as shown by studies of her skin fibroblasts. Erythroid and granulocytic colonies generated in methylcellulose assays of fresh marrow from this patient were all Ph-positive and expressed the enzyme G6PD-A as found in the mature blood cells. In assays of 4- and 6-week-old long-term cultures, numbers of progenitors present were low and most were Ph-positive, but some (2/11) were Ph-negative. G6PD analysis showed that a similar proportion was not part of the neoplastic clone (4/30 were G6PD-B and 26/30 were G6PD-A). Thus, long-term marrow culture in conjunction with G6PD analysis was able to reveal the presence of a nonclonal hemopoietic progenitor population in a CML patient where such cells would not otherwise have been detected. This approach may offer a novel strategy for the demonstration and selection of populations of normal hemopoietic progenitors in other patients where the malignant cells cannot be distinguished by chromosome analysis.