A modified polysaccharide-ditrizoate gradient centrifugation method is described. One hundred ml aliquots of marrow mixed 1:1 with medium is layered on 70 ml of polysaccharide-ditrizoate solution of specific density 1.077 in 5 x 19 cm flasks and centrifuged at 400 x g for 20 minutes at 20 degrees C. Interphase cells are collected and used for the transplant. The procedure is closed which reduces the danger of contamination, simple and takes between 2 and 3 hours. Two allogeneic HLA-identical transplants were performed using this technique and both resulted in full hematopoietic reconstitution and long term survival.