491 cpm/pmol), 15%(v/v) glycerol and 150mMKC1. After incubation at 37 C for 60 minutes, the radioactive DNA product was collected on a DEAE-cellulose paper (DE8 1) disc as described by Lindell et al.(1970) 4). In the first screening, 6 actinomycetes, 1 fungus and 12 mushroomstrains were found to produce inhibitors, and of these, the product of a species of the Polyporaceae family appeared to be most promising. Theactive principle of the Polyporaceae strain waspurified byw-hexane extraction and alumina column chromatography followed by partitioning in chloroform-water(pH 2).

Unexpectedly, the active principle was identified by 1H, 13C and^ H-^ C COSYNMRand FAB-MSas linoleic acid, an essential fatty acid in higher organisms. Toestablish the secondly screening system, the effects of linoleic acid (commercially-purchased 18: 2 A9~ 12 cis) on the in vivo growth of cultured mammaliancells, adults and progeny of Drosophila melanogaster and an Arabidopsis plant were investigated. For mammaliancell