Dextran sedimentation with centrifugation and semicontinuous flow centrifugation was used to concentrate marrow stem cells from collections of human bone marrow. With the dextran sedimentation method, recovery of the myeloid stem cells was 92 percent in 35 percent of the original volume, but contamination by mature cells was high. In the semicontinuous flow centrifugation method the recovery of myeloid stem cells was 82 percent in 15 percent of the original volume. Contamination with platelets, erythrocytes and neutrophils was reduced to 20.4, 29.6, and 38.6 percent respectively of the original values. It was concluded that semicontinuous flow centrifugation provides a comparatively convenient and efficient method for concentrating large collections of human bone marrow cells in a closed system.