Methyl viologen (MV) (20-150 mu M), a generator of superoxide anion (O-2(-)), but not hydrogen peroxide (H2O2) (10 mu M-2 mM) triggered the formation of cryptotanshinone (a phytoalexin) in cultures of both crown galls and hairy roots of Salvia miltiorrhiza. MV also inhibited the biomass formation and decreased the contents of phenolic acids in both cultures whereas H2O2 did not. In addition, MV and yeast elicitor induced cryptotanshinone formation synergistically only in crown gall cultures. Treatment of the cultures with 3.3 mu M diphenylene iodonium, an inhibitor of NAD(P)H oxidase, did not exhibit any detrimental effect on the yeast elicitor-induced cryptotanshinone formation in hairy root cultures whereas 1 mu M diphenylene iodonium was inhibitory on yeast elicitor-induced cryptotanshinone formation in crown gall cultures.