Some multilineage hemopoietic colonies contain, in addition to myeloid cells, T lymphocytes. These proliferate extensively in liquid suspension culture under the influence of a T cell growth factor provided by phytohemagglutinin-T cell-conditioned medium (PHA-TCM). The clonal origin of these myeloid and lymphoid components was investigated by determining the glucose-6-phosphate dehydrogenase (G6PD) isoenzyme types of multilineage colonies grown from peripheral blood of 4 G6PD heterozygous normal volunteers. The G6PD assay is sufficiently sensitive to detect enzyme concentrations contributed by as few as 30 granulocytes and erythroblasts, 4–6 megakaryocytes, 2–3 macrophages, and 50–100 T cells. T cell components can be detected even if myeloid cells are present in 10–20-fold excess. A small number of multilineage colonies with T cells produced a single G6PD isoenzyme on direct analysis and after expansion in liquid culture. This observation supports the view of a common progenitor for myeloid and lymphoid cells in the peripheral blood of normal adults.