Abstract: Human cell lines have been obtained which elaborate colony stimulating activity (CSA) for marrow cells, i.e., which enable cells to be grown in vitro for use in bioassay methods in diagnostics and therapy. One cell line, GCT, was obtained from a lung metastasis of a fibrous histiocytoma and another, RC4 from a monocyte enriched fraction of normal blood. These cell lines provide a continuous source of large quantities of conditioned medium from purification of CSA and is more potent in stimulating human narrow growth in vitro than conditioned mediums heretofore available, i.e., human embryo kidney cell or human leukocyte feeder layers, having finite life spans in culture.