The multipotent hemopoietic stem cell has fastidious growth requirements in vitro. Traditionally, phytohemagglutinin-stimulated leukocyte conditioned medium has been used to supply the undefined growth factors required for culture of the human multipotent hemopoietic progenitor. We describe the use of medium conditioned by the bladder carcinoma cell line, 5637, to replace PHA-LCM in CFU-GEMM cultures and show that the properties of this conditioned medium closely mimic those of PHA-LCM in two separate CFU-GEMM culture systems.