Grand fir callus tissue, obtained from debarked sapling stems and cultured on modified solid Murashige and Skoog medium, when transferred to medium lacking growth regulators or glutamine, or containing autoclaved extracts of Trichosporium symbioticum or Penicillium brevicompactum, or chitosan, yeast extract or pectinase, displayed up to an eight-fold increase in the level of monoterpene cyclase activity compared to controls. The activity induced in the calli was principally limonene cyclase, not pinene cyclase which is the major wound-inducible monoterpene synthase in grand fir sapling stems. Mechanical damage to callus tissue and treatment with filter-sterilized extracts of wounded stem had no marked effect on cyclase activity levels. The regulation of induced monoterpene biosynthesis in grand fir calli is different than that observed in wounded saplings; the complement of cyclases induced in cultured cells resembles more closely the constitutive pattern of activities present in intact stems.