Stem cell concentrates were prepared from mouse bone marrow by treating the marrow donors with a vinblastinenitrogen mustard regimen and subjecting the marrow suspensions to repeated density separation over albumin gradients. An enrichment of the colony-forming units (CFU) content by a factor of up to 30 as compared to normal mouse bone marrow was obtained, which corresponds to a calculated stem cell content of over 20%. Using an adapted May-Grünwald-Giemsa staining method and electron microscopic techniques, the morphologic characteristics of a candidate stem cell have been defined in these enriched cell populations. These criteria have been applied to count candidate stem cells in preparations varying over a 100-fold range in their CFU content, and a fair correlation between the two entities was found. The candidate stem cell is shown to be quite distinct from the so-called "small lymphocyte."