Interleukin-1 (IL-1) may play a critical role in immune and inflammatory responses in inflamed gingiva, and it is synthesized by a wide variety of host cells. In this study, we examined the regulatory effects of various cytokines on bioactive membrane IL-1 and intracellular IL-1α production in cultured human gingival fibroblasts (HGF). Recombinant human (rh) IL-lp stimulated membrane IL-1 activity, which was mainly attributed to IL-la. rhIL-1β and rh tumor necrosis factor (TNF)-a stimulated HGF to produce intracellular IL-la, whereas rh interleukin-6 (IL-6), rh interleukin-4 (IL-4), and rh interferon (IFN)-γ did not do so. Intracellular IL-la production induced by rhIL-1β or rhTNF-a may be partially related to protein kinase C (PKC) activation, because rhIL-1β or rhTNF-a-induced intracellular IL-la production was stimulated by pre-treatment with 12-o-tetradecanoylphorbol 13-acetate (TPA), a PKC activator, but was suppressed by the pre-treatment with 1-(5-isoquinoline-sulfonyl) -2-methylpiperazine dihydrochloride (H-7), which is a PKC inhibitor. rhIL-4 inhibited rhIL-1β- or rhTNF-a-induced intracellular IL-la production, but rhIL-6 had no effect on this production. Pre-treatment with rh IFN-γ remarkably enhanced intracellular IL-la production induced by subsequent treatment with rhIL-1β or rhTNF-a. Simultaneous treatment with rhIFN-γ and rhIL-1β inhibited rhIL-1β-induced intracellular IL-la production, but co-treatment with rhIFN-γ and rhTNF-a enhanced rhTNF-a-induced intracellular IL-la production. These results suggest that in inflamed gingiva, pro-inflammatory cytokines such as IL-1β and TNF-a may induce bioactive intracellular IL-la production in human gingival fibroblasts and that this production can be differentially modulated by T-cell-derived cytokines such as IFN-γ or IL-4.