The adhesive properties of clonogenic haemopoietic progenitor cells have been assessed by allowing them to bind to cultured marrow-derived stromal layers, stromal cell lines or bare plastic surfaces. We have compared the adhesive characteristics of murine marrow repopulating cells, spleen colony-forming cells (CFU-S) and granulocyte-macrophage colony-forming cells (GM-CFC) and of human blast colony-forming cells (Bl-CFC). The data indicate, first, that murine marrow repopulating cells bind to plastic and to stromal layers. Second, the CFU-S compartment contains three sub-populations: (a) CFU-S that bind to plastic and to stroma; (b) CFU-S that bind to stroma but not to plastic; (c) CFU-S that do not bind to stroma or plastic. Third, that more mature GM-CFC are not adhesive in these tests; and fourth, that recognitive events between stem cells and marrow stroma can operate across species barriers. Adhesion to both marrow-derived stroma and plastic appears to be a characteristic of haemopoietic stem cells that is lost or modulated as cells differentiate.