Expression of the light-inducible lipA gene in Arthrobacter photogonimos by photodynamic compounds and visible light was inhibited by washing cells with 1 M KCl. Addition of cell surface extract to KCl-washed cells restored the induction. Washing cells with 1 M MgCl2 removed a 14-kDa polypeptide and concomitantly caused expression of lipA gene without photodynamic treatment. The purified 14-kDa polypeptide inhibited photodynamic induction of lipA gene. These results indicated that regulation of lipA gene expression occurred at the cell surface and involved positive and negative factors, probably through a signal transduction system.