We have screened several chinese medicinal herbs for the presence of antifibrotic agents. An aqueous extract of Salviae miltorrhizae Radix was found to inhibit collagen secretion by human skin fibroblasts without affecting DNA or noncollagen protein synthesis. We have subsequently purified the material exhibiting the inhibitory activity and identified it as magnesium lithospermate. From its chemical structure this compound was predicted to be an inhibitor of the post-translational modifying enzymes prolyl and lysyl hydroxylases in collagen biosynthesis. Accordingly, it decreased the extent of prolyl and lysyl hydroxylations in collagen by approx. 50%. Added to cell extracts it inhibited both prolyl and lysyl hydroxylase activities, but only lysyl hydroxylase activity when added to intact cells. Oral administration of this compound to mice led to a significant reduction of prolyl hydroxylation in newly-synthesized skin collagen. This naturally-occurring compound thus offers a potential means for treating fibrotic diseases, such as systemic scleroderma and keloid.