We have shown that bone marrow microvascular endothelial cells (BMEC) support growth and differentiation of haemopoietic progenitors in vitro by elaboration of haemopoietic cytokines. Since generation of eosinophils can be observed in these coculture experiments, and BMEC do not produce interleukin (IL)-3, we evaluated BMEC for expression of IL-5, a specific growth factor for the eosinophilic lineage. Using RT-PCR, IL-5 mRNA was expressed by BMEC after stimulation (12 h) with lipopolysaccharide (LPS), IL-1, IL-2 and phorbol myristate acetate (PMA), but not by resting BMEC, after stimulation with TNF-α or interferon (IFN)-γ. Moreover, IFN-γ suppressed expression of IL-5 in response to LPS and IL-2. The identity of the PCR products was confirmed by restriction enzyme digestion, which resulted in fragments of the predicted size. T lymphocytes were not present in the endothelial cultures as demonstrated by absence of CD2 mRNA. Using a sensitive (1 pg/ml) ELISA assay, IL-5 was detected after 48 h incubation of BMEC with IL-2 (4.1 pg/106 cells) or with a combination of LPS and IL-2 (4.8 pg). However, the number of eosinophils generated after 4 weeks coculture of CD34+ haemopoietic cells with BMEC was not increased by addition of IL-2. RT-PCR revealed that BMEC in coculture with haemopoietic cells expressed IL-5 even without addition of exogenous cytokines or stimulating agents. In conclusion, expression of IL-5 by BMEC can be stimulated by cytokines (IL-1, IL-2), LPS, PMA, and coculture with proliferating haemopoietic cells. Thus, BMEC may support proliferation and differentiation of eosinophils in the bone marrow. IFN-γ represents a cytokine with an inhibitory effect on IL-5 expression by BMEC. In addition, eosinophilia in response to circulating IL-2 or bacterial products (LPS) in vivo may be partially mediated by BMEC or vascular endothelium.