Purified human fetal BM CD34⁺ thy-1⁺ cells from each sample (average 450 000 to 500 000 cells) were divided into 3 fractions. One fraction (approximately 400 000 cells) was used to inject 40 SCID-hu mice (20 thy/liv mice and 20 bone-model mice; 10 000 cells per graft). The other 2 fractions (about 10 000 cells for each fraction) were used to initiate stromal-based cultures (positive control) and SCM-based cultures (300 cells per well in two 24-well plates for each system). Three weeks later, ex vivo expanded CD34⁺ thy-1⁺ cells were sorted separately from both stromal-based cultures (approximately 1 million CD34⁺thy-1⁺ cells) and SCM-based cultures (approximately 2.5 million CD34⁺ thy-1⁺ cells), and cells from each culture system were injected into 40 SCID-hu mice (again, 20 thy/liv mice and 20 bone-model mice; 10 000 cells per graft). The frequency of donor-reconstituted animals and percentage of donor-derived hematopoietic cells in the reconstituted animals were determined at 3 to 4 months after stem cell injection. These data were compiled from 3 independent experiments using different donor BM samples. The frequency of reconstituted animals is presented as the total number of reconstituted animals divided by the total number of animals used in the experiments. The percentage of donor-derived cells represents the mean ± SEM of all reconstituted animals from the same experiments.