Major advances in our understanding of human hematopoiesis have come from the development of semisolid in vitro culture techniques for the detection of progenitor cells capable of colony formation (see Chapter 28 , this volume). However, colony assays select for hematopoietic precursors committed to a specific lineage(s) and, therefore, are of limited value in the assessment of very early progenitor cells. Also, they do not provide a means of assessing the influence of microenvironmental cells deemed essential for maintaining hematopoiesis in vivo (1). Furthermore, for the assays to be valid, cells must be plated at a sufficiently low concentration to ensure clonality (a condition in which each colony has arisen from a single progenitor cell). Consequently, cell-cell interactions that may be important in exerting regulatory effects on hematopoiesis are likely to be minimal.