The migration of hematopoietic progenitor cells enriched for a primitive phenotype. Progenitor cells were isolated on the basis of lack of expression of lineage markers and expression of Sca antigen or c-Kit antigen. Sca⁺ indicates Lin^– Sca⁺ cells; Kit^– indicates Lin^– Sca⁺ c-Kit^– cells; and Kit⁺ indicates Lin^– Sca⁺ c-Kit⁺ cells. A Boyden chamber assay was used to measure cell migration in response to SDF-1 (A) or LPA and S1P (B) present in either the top well (shown as LPA/NA on the x-axis), the bottom well (NA/LPA), or both. Results are the mean ± SEM of 3 or more experiments. Statistical analysis was by Student paired, 2-tailed t test. The actual number of cells migrating in the 30-μL volume varied between 16 × 10³ for the Kit^– cells with SDF-1 and 1 × 10³ for the Kit⁺ cells with no addition.