Using autologous serum as medium, localized leukocyte mobilization into a plastic chamber at the site of mechanical abrasion was quantified in 5 patients with florid CML, one patient with chronic eosinophilic leukemia in the florid state, 3 patients (5 studies) with CML and partial response (remission or relapse), 6 patients with CML in good remission, and in 7 patients with CML in die blastic phase. The cumulative total number of cells was expressed as total leukocyte mobilization (TLM) × 106/cm2 at 24 hrs. Leukocyte clearance/cm2 in 24 hrs. was expressed in milliliters of blood and was obtained by dividing TLM (corrected for non-granulocytes) by the absolute granulocyte count in the peripheral blood. Although the median TLM was found to be elevated in CML with florid disease (130 × 106/cm2 at 24 hrs.) and in CML with partial remission and relapse (104 × 106), it fell to 74 × 106 in 6 patients in remission. This is to be compared with the median TLM for 46 normal subjects (76 × 106/cm2 at 24 hrs.). Median leukocyte clearances showed the lowest values in CML in the florid state (1.0 ml/cm2 in 24 hrs), improving to 4.4 ml when a partial response was present, and to 9.3 ml in remission. This compares with 19.7 ml/cm2 in 24 hours for 46 normal subjects. The median TLM in 7 patients with CML in blastic phase was 14 × 106/cm2 at 24 hrs., and the leukocyte clearance was reduced to 1.4 ml. These data suggest a defect in cell-migration in all stages of CML, which is inversely correlated with clinical state. The data reinforce the reported prolongation of circulation time of leukocytes in CML, and suggest a mechanism for this in the impaired ability of CML leukocytes to migrate across the vessel wall.