Effect of indigocarmine (IC) exposed to ultraviolet light on the mutagenicity of Trp-P-1 was examined. Enhancing effect of IC on the mutagenicity of Trp-P-1 was decreased with elongation of period of ultraviolet irradiation. The enhancing effect of IC was completely removed through the ultraviolet irradiation for 8.5h. Isatin, a derivative of indigoid compound, was also found to increase the reverse mutation induced by Trp-P-1 in the presence of microsomal activation system. Further, the addition of ascorbic acid to the reaction mixture enhanced the mutagenicity of Trp-P-1 by isatin depending on the concentration of ascorbic acid. Conversely, the addition of alph-tocopherol showed no enhancing effect.