Hematopoietic stem cells form colonies in the spleens of lethally irradiated mice. The number of colonies is indicative of the number of stem cells present in the inoculum. In this article, a technique is presented for effecting a seven-fold enrichment of spleen colony-forming cells. Bone marrow cells are first segregated into "red" and "white" cell populations by centrifugation on Ficoll-Paque...