A new class of cell surface antigens are described which are expressed on cells within a particular differentiation pathway. These antigens, termed cell-lineage-specific antigens, are shown to be distinct from differentiation antigens and from histocompatibility antigens. The presence of these antigens was demonstrated by raising antisera against terminally differentiated hemopoietic cells such as platelets, thymocytes, and macrophages and showing cross-reaction with the pluripotent stem cells from which these cells were derived. Quantitative absorption studies of each antiserum showed the antigens to be largely cell-lineage-specific. For example, the anti-stem cell activity in anti-platelet serum was not absorbed out with thymocytes or macrophages from the same mouse strain but was removed by absorption with platelets. Absorption of each antiserum with nonhemopoietic mouse tissues such as brain, kidney, liver, and testis did not reduce anti-stem cell activity. Thus, each antiserum was shown to be tissue specific and species specific. Hemopoietic cells from mouse strains other than CBA absorbed out most of the anti-stem cell activity indicating cell-lineage-specific antigens to be common to the mouse strains tested.