A new assay for the long-term functional capacity of hemopoietic stem cells is reported. Stem cell function in each donor of a particular genotype is assayed by mixing its marrow cells with a constant number of marrow cells from a donor with distinguishable hemoglobin and measuring the relative ability of each donor to populate stem-cell- depleted recipients. For example, cells from many different B6 donors may be assayed by mixing them with a constant number of WBB6F1 cells from a single pool and injecting them into irradiated WBB6F1 recipients. As the ratio of B6 to WBB6F1 marrow cells increases from 0:1 to 3:1 the percentage of B6 hemoglobin increases in a linear fashion. This is also found with WB and WBB6F1 or CBA and B6CBAF1 mixtures. Correlation coefficients between the percentage of hemoglobin of one donor type and the ratio of cells in the mixture of that type ranged from 0.78 to 0.98 in single experiments, and were 0.68 and 0.75 using data pooled from several experiments.