In order to determine if mononuclear cells may be secreting factors capable of modulating fibroblast growth, the in vitro proliferative response of fibroblasts to cytokines known to be secreted by mononuclear cells was measured, using both growth arrested and proliferating cells. Of the cytokines tested, which included interleukin-1 (IL-1), interleukin-2 (IL-2), interleukin-3 (IL-3), interleukin-4 (IL-4), interleukin-6 (IL-6), interferon-alpha (IFN-alpha), interferon-gamma (IFN-gamma), transforming growth factor-alpha (TGF-alpha), transforming growth factor-beta (TGF-beta), platelet derived growth factor (PDGF), and tumor necrosis factor-alpha (TNF-alpha), only TNF-alpha and PDGF had demonstrable growth factor activity. Neither IL-1 alpha nor beta showed any true growth factor activity but were able to enhance the replication of already proliferating cells. No inhibition of proliferation was noted by any of the cytokines with the exception of TNF-alpha and TGF-beta. TNF-alpha in doses greater than 500 ng/ml caused fibroblast death, probably by a prostaglandin related mechanism as fibroblasts remained viable, although non proliferative, when assayed in the presence of indomethacin, a known inhibitor of prostaglandin E2 (PGE2) synthesis. TGF-beta was inhibitory to proliferation at doses greater than 100 ng/ml, while fibroblasts remained viable. This effect was not influenced by indomethacin and hence is unlikely to be PGE2 related.