The antifungal activity of radicicol against Mucor flavus IFO 9560 was investigated. Radicicol induced bursting of spores during germination and morphological changes of the mycelial tip such as overbranching and swelling during exponential growth. In addition, radicicol showed a dose-dependent inhibitory effect on spore germination. Radicicol also inhibited the incorporation of radioactive precursors into DNA, RNA, protein, and chitin fractions by 20-30%, but not into the lipid fraction. There were no inhibitory effects on either endogenous or exogenous cellular respiration. Moreover, leakage of UV-absorbing, phenol sulfate-positive, or folin reagent-positive materials from the mycelia was not observed at an early stage of growth inhibition. On the other hand, kinetic studies of chitin synthase in the untreated mycelia revealed that radicicol noncompetitively inhibited the enzyme at Ki of 87 microM. Furthermore, upon incubation of the normal mycelia with radicicol in 50 mM KH2PO4-NaOH buffer (pH 6.5) containing 10 mM MgCl2, chitin synthase from the mycelia was inactivated gradually at first, and completely after 24-h incubation. These results suggested that radicicol exhibits the antifungal activity by disturbing cell wall biosynthesis through the inactivation of chitin synthase. However, at an early stage of growth inhibition, radicicol was thought to affect cellular function including nucleic acid and protein syntheses, in addition to the reversible noncompetitive inhibition of chitin synthase.