CD34+ BM cells form a heterogenous population of primitive stem cells and more mature progenitors committed to different lineages of differentiation. By combining CD45 expression with SSC, it is possible to separate immature cells from more differentiated BM cells, and, by three-colour flow cytometry, analyse the antigens expressed in various subsets of cells. In this paper we show that in the normal BM at least four distinct CD34+ cell populations can be identified by their different patterns of CD45 expression and SSC. The most immature CD34+ cell population (0.1% of the BM cellularity) lacked all signs of lineage commitment and was CD45RA negative and only weakly CD45 positive. With increasing expression of the CD45 antigens, a second CD34+ population (0.2% of the BM cellularity) was formed expressing mainly primitive lymphatic antigens. However, 30% of the cells co-expressed B-cell line antigens and myeloid antigens. Cells committed to the myeloid cell line lost B-cell line antigens, gained CD45 antigen expression and SSC and formed two CD34+ cell populations (0.2% and 0.1% of the BM cellularity, respectively) differing only with respect to the pattern of myeloid antigen expression and SSC characteristics. Similarly, differentiation along the lymphatic pathway implicated down-regulation of myeloid antigens, loss of the stem cell antigen and immature lymphatic antigens and gain of CD45 expression and mature lymphatic antigens.