The cloning of hemopoietic cells in semisolid medium began some 12 years ago, and when colonies of T = and B = lymphocytes were grown by several groups last year, the last major subclasses of hemopoietic cells had been successfully cultured in vitro. The time seemed appropriate to write a short general account of this exciting new area of hematology, emphasizing particularly the potential value of these new techniques for direct studies on patients with leukemia and allied diseases. I had hoped to refer to, or at least record, every paper published in this field, but the literature proved to...