In vitro expansion of haemopoietic progenitor cells (HPC), lineage-specific differentiation, and gene transfer are all based on in vitro culture systems using haemopoietic growth factors (HGF). A close control of the actual culture conditions, however, is difficult due to secondary mediators secreted by the heterogenous population of mature and immature cells in culture. Although monocytes and granulocytes have already been identified as active producers, this study specifically addressed the role of CD34+ progenitor cells in this respect. Using an immunostaining method that enables simultaneous detection of cytokines and phenotype, 56 +/- 6% CD34+ peripheral blood progenitor cells (PBPC) were found to contain cytoplasmic IL-8 after stimulation with phorbol myristate acetate + ionomycin for 90 min. 19 +/- 40%, stained positive after TNF-alpha induction (20 h), and 7 +/- 1% expressed IL-8 in the presence of culture medium alone. Intra-cytoplasmic TNF-alpha and IL-1beta were detected at lower frequency, and < 1% of CD34+ cells expressed IL-1ra or IL-6, whereas IL-1alpha, IL-10 and G-CSF were not detected. Thus, CD34+ HPC are able to synthesize chemo- and cytokines that may operate in an auto- or paracrine manner to modulate in vivo as well as in vitro growth and differentation of haemopoietic cells.