Female adult New Zealand white rabbits were anesthetized using a combination of xylazine and ketamine administered intramuscularly. Bone marrow was aspirated aseptically from the humerus of femur using an 18-gauge Rosenthal pediatric needle. Marrow was obtained from six animals per hour using this technique. Recovery was rapid and no infection or loss of limb function was observed over the course of more than 100 aspirations. Approximately 2 x 10(8) mononuclear cells were obtained from 0.5 ml of aspirated marrow. Marrow samples were fractioned using isopynic single step density gradients. A 10-fold enrichment of blast cells was achieved using a density of 1.077 g/ml.