Recombinant DNA technology has made possible the analysis of cytokine gene expression in both in vitro and in vivo studies of human hematopoietic malignancies and has facilitated the production of large amounts of recombinant cytokines. This development has led to advances in our understanding of the role of aberrant cytokine production in these diseases. These results support the concept of autocrine stimulation of leukemic growth, for instance, in multiple myeloma and acute myelogenous leukemia, and may lead to new therapeutic concepts such as the application of antibodies directed against these cytokines. Availability of recombinant cytokines has also allowed clinical testing in settings of disease- or therapy-related neutropenias and anemias, and particularly GM-CSF, G-CSF, and EPO have proven efficient in this respect. Thus, there is the prospect of more dose-intensive chemotherapy protocols as well as combinations of different cytokines that may prove more effective than application of a single compound.