The colony-forming unit culture (CFU-C) in the thin-layer agar colony technique is considered to be representative for hemopoietic stem cells (HSC), according to our studies in mouse and monkey bone marrow. Using this in vitro assay as a guide, stem cell concentrates were prepared from monkey and human bone marrow by repeated density gradient centrifugation. The number of CFU-C could be enriched up to 70-100-fold. In such concentrated CFU-C suspensions, a cell, morphologically identical with the hemopoietic stem cell in the mouse (MSCLC, mouse stem cell-like cell) was frequently observed, using a May-Grünwald-Giemsa (MGG) staining method and electron microscope techniques. In MGG-stained preparations, the MSCLC superficially resembles the small lymphocyte; therefore, a staining method has been described, the polychrome procedure, by which both cell populations could be clearly distinguished. Since a fair correlation exists between the number of MSCLC and the number of CFU-C in a variety of primate hemopoietic suspensions, we concluded that the MSCLC might be a good candidate for being the HSC in monkeys and man.