Transendothelial migration in vitro. A total of 5 × 10⁵ cells (purified PB CD34⁺ progenitors, different cell lines, or primary leukemic blasts) were added to the upper chamber of the transmigration system. After 10 hours, transmigrated cells recovered from the lower chamber were enumerated. Due to the detection limit of this assay, migration of >1% could be quantified reliably. Spontaneous migration (addition of control medium to the lower chamber) was compared with SDF-1–induced migration (addition of SDF-1–containing conditioned medium to the lower chember). CD34⁺ progenitors, CD34⁻ leukemic (HL60), and lymphoma (OCI-Ly8, DOHH-2) cell lines showed significant migration, in contrast to CD34⁺ leukemic cell lines. SDF-1–induced migration of primary AML blasts was variable.