The discovery that mast cells are a potential source of cytokines has suggested new ways in which mast cells can act in immunological and inflammatory responses. In this study we have used the HMC-1 cell line as a model for human mast cells to study the constitutive and inducible mRNA expression of interleukins, colony-stimulating factors, interferons, tumour necrosis factors alpha and beta, tumour growth factor beta and platelet-derived growth factor A and B. We found that HMC-1 cells constitutively expressed mRNA for TNF-alpha and TGF-beta, and a low level of M-CSF. After treatment with the phorbol ester TPA or the calcium ionophore ionomycin expression of several cytokines, i.e. IL-1 beta, IL-3, IL-6, GM-CSF, TNF-beta and PDGF-A, could be detected. Both TPA and ionomycin induced the same set of cytokines, but the effect of TPA was more prominent. The relative induction was calculated to be 70X for IL-1 beta and IL-3, 30X for GM-CSF and PDGF-A and 3 - 10X for IL-6, M-CSF and TNF-beta. This study shows that human mast cells have the capacity to express not only cytokines mediating an immune response but also cytokines affecting other cell types, e.g. fibroblasts and endothelial cells, involved in later steps of the inflammatory response.