The effect of different types of plant growth retardants on fungal gibberellin (GA) formation has been studied in cultures of Gibberella fujikuroi and Sphaceloma manihoticola. Quaternary ammonium compounds (chlormequat chloride, mepiquat chloride, Amo-1618), triazoles (uniconazole and several experimental compounds), and the norbornanodiazetine tetcyclacis inhibited GA biosynthesis in both fungal species. Concentrations between 2 x 1iO`and 10-M were required for a 50% inhibition of the production of gibberellin A3 in Gibberella fujikuroi and of giberellin A4 in Sphaceloma manihoticola. The formation of other prominent GAs was affected at a similar degree of intensity. Tetcyclacis was the most active compound in both fungi. Compared to the growth retardants mentioned above, the biological activity of chlorphonium chloride was low. The acylcyclohexanediones prohexadione and LAB 198 999 had virtually no activity. Most likely, this lack of activity is due to a rapid metabolism of the compounds in the cultures. For the triazole-type compounds and tetcyclacis, a relatively distinct correlation exists in their ability to inhibit GA formation in fungal cultures, to block ent-kaurene oxygenase in a cell-free system, and to reduce shoot growth of rice seedlings. Due to differences in their metabolic fate and species specificities, such conclusions cannot be made for the other compounds.Plant growth retardants play an important role in agriculture and horticulture in reducing unwanted shoot elongation (21). At present, five major groups have been recognized. Ethylene-releasing compounds such as ethephon reduce shoot elongation mainly by inhibiting cell division. Daminozide most likely acts by causing a more rapid inactivation of GAs1 and by inhibiting GA translocation (31). Quaternary ammonium and phosphonium compounds (e.g. CCC, mepiquat chloride, Amo-1618, and chlorphonium chloride), compounds with a nitrogen-containing heterocycle (e.g. ancymidol, tetcyclacis, paclobutrazol, uniconazole, and inabenfide), and acylcyclohexanediones (e.g. prohexadione, cimectacarb, and LAB 198 999) inhibit the biosynthesis of GAs. It is known that representatives of the latter three groups block, respectively, the cyclization of geranylgeranyl pyrophosphate via copalyl pyrophosphate into ent-kaurene, the oxidative reactions from ent-kaurene to ent-kaurenoic acid, or 3,B-hydrox-