Norharman enhanced the known mutagenicity of 2-acetylaminofluorene derivatives in the Salmonella test system. The mutagenicities of 2-acetylaminofluorene, 2-aminofluorene, and N-hydroxy-2-acetylaminofluorene were enhanced by norharman only when rat liver microsomal enzymes were added, whereas the mutagenicity of N-acetoxy-2-acetylaminofluorene was increased in the absence of microsomal enzymes. Harman also increased mutagenesis, althouth less so than norharman.