A number of recent advances have resulted in the definition of human pluripotent hemopoietic progenitors that can be considered candidates for stem cells. These cells may be CD34 positive and HLA-DR negative. They are capable of producing colonies composed of cells with blast-like morphology and re-establish long term bone marrow cultures when placed on irradiated stromal layers. These cells appear to interact at short range with stromal cells and become responsive to signals that initiate their proliferation and differentiation. These signals can be provided by interleukins produced by various auxiliary hemopoietic cell populations and may act over long distances. In addition, these interleukins may be aberrantly produced by certain malignant hemopoietic cells. This may result in paramoplastic auto- or paracrine growth promotion of the underlying malignancy and occasionally in the development of paraneoplastic syndromes.