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T he production of interleukin 1beta (IL-1beta) by human hematopoietic stem/progenitor cells was studied to explore the concept that these cells are not merely responders to stimuli from their microenvironment, but can themselves produce a powerful biomodulator. Cells with a CD34+ CD45RA(lo) CD71(lo) phenotype were purified from human umbilical cord blood and cultured one per well in serum-free medium with a mixture of cytokines. Cells that had divided over 2-5 d to form doublets were identified and the daughter cells were studied individually. 91% (460/506) of daughter cells had clonogenic potential. Analysis of these individual daughter cells by reverse transcription-polymerase chain reaction showed that 29% of them (14/48) were positive for IL-1beta mRNA. One of the cells that was strongly positive for IL-1beta mRNA had a sibling that generated 366,000 cells of multiple lineages after 14 d. IL-1beta converting enzyme mRNA, which is necessary to produce IL-1beta, was also detected by reverse transcription-polymerase chain reaction at the single-cell level. Moreover, enzyme immunoassay for mature secreted IL-1beta in culture supernatants demonstrated the production of IL-1beta protein by these cells. This was confirmed by fluorescent immunostaining of the cells for human IL-1beta which showed a significant portion of positive cells. Taken together, the results demonstrate the capacity of early hematopoietic cells to synthesize IL-1beta. The capacity of human hematopoietic stem/progenitor cells to produce IL-1beta may be involved in regulation of their proliferation and differentiation under certain circumstances and dysregulation of this process may be modified in leukemogenesis.