The glucose-6-phosphate dehydrogenase (G-6-PD) types and chromosomes of hematopoietic and other tissues were determined in a woman with agnogenic myeloid metaplasia. The patient was heterozygous at the X- linked G-6-PD locus so that both B and A isoenzymes were found in nonhematopoietic cells. In contrast, only one G-6-PD type was found in granulocytes, red cells, and platelets. She also had a distinctive chromosome abnormality in blood cells but not in other tissues. These results indicate that agnogenic myeloid metaplasia is a disorder of a pluripotent stem cell and provide strong evidence that it is of clonal origin. In contrast to blood cells, the patient's cultured marrow “fibroblasts” had normal chromosomes and both B and A G-6-PD types, suggesting that the marrow fibrosis is a secondary abnormality. Thus, at least in this case of agnogenic myeloid metaplasia, the hematopoietic cell proliferation appears to be clonal, and, by inference, possibly neoplastic, whereas the marrow fibrosis is probably not clonal, and therefore appears to be secondary.