The role of p38 mitogen-activated protein kinase (MAPK) in responses of human fibroblasts and vascular endothelial cells to IL-1 was investigated by use of a pyridinyl imidazole compound (SB 203580), which specifically inhibits the enzyme. SB 203580 inhibited (50% inhibitory concentration approximately 0.5 microM) IL-1-induced phosphorylation of heat shock protein 27 (an indicator of p38 MAPK activity) in fibroblasts without affecting the other known IL-1-activated protein kinase pathways (p42/p44 MAPK, p54 MAPK/c-Jun N-terminal kinase and beta-casein kinase). SB 203580 significantly inhibited IL-1-stimulated IL-6, (30 to 50% at 1 microM) but not IL-8 production from human fibroblasts (gingival and dermal) and umbilical vein endothelial cells. IL-1 induction of steady state level of IL-6 mRNA was not significantly inhibited, which is consistent with p38 MAPK regulating IL-6 production at the translational level. SB 203580 strongly inhibited IL-1-stimulated PG production by fibroblasts and human umbilical vein endothelial cells. This was associated with the inhibition of the induction of PGH synthase-2 protein and mRNA. SB 203580 also inhibited the stimulation of collagenase-1 and stromelysin-1 production by IL-1 without affecting synthesis of the tissue inhibitor of metalloproteinases (TIMP)-1. SB 203580 prevented the increase in collagenase-1 and stromelysin-1 mRNA stimulated by IL-1. In a model of cartilage breakdown, short-term IL-1-stimulated proteoglycan resorption and inhibition of proteoglycan synthesis were unaffected by SB 203580, while longer term collagen breakdown was prevented. It is concluded that 1) p38 MAPK plays an important role in the regulation of some, but not all, responses to IL-1, and 2) it is involved in the regulation of mRNA levels of some IL-1-responsive genes.