Lysyl oxidase (LO), an extracellular enzyme catalysing the first step of collagen and elastin cross-linking, is transiently expressed by myofibroblasts during fibrosis. A cell model with features of myofibroblast was thus established for studying the regulation of LO. Two clones of the 3T6 fibroblast cell line were selected because 1) they produced a relatively high steady-state level of the three lysyl oxidase mRNAs with the same relative ratio similar to fibrotic tissue and 2) they stably displayed certain features of myofibroblast (a-smooth muscle actin cytoskeleton, bundles of cytoskeletal filaments beneath the cytoplasmic membranes). These clones synthesized predominantly type I collagen fibers and a small amount of type III collagen. Neither type IV collagen nor elastin were observed. The cloning and sequencing of 2,073 bp of the mouse Balb/C LO promoter was performed, allowing the identification around the initiation of transcription of consensus sequences which are found on the COL1 promoters. A series of deletion constructs containing the LO 58-flanking region ligated to the luciferase gene were transiently transfected into 3T6-5 fibroblasts. The region allowing the maximal activity was found between positions 2416 to 2192, while the more upstream region negatively regulated the promoter. The 2898 to 2865 sequence (called LOcol1) displayed 79% of homology with a conserved sequence of murine, rat, and human COL1A1 promoters. This sequence participated to the binding of several nuclear factors within a region (2970 to 2784) allowing 50% of inhibition of the LO promoter. Therefore, the level of LO transcription is regulated in 3T6-5 fibroblast by positive and negative cis-acting regulatory elements which might have common features with the COL1A1 promoter.