The activity of triazole fungicides towards Botrytis cinerea was investigated in vitro (radial growth on fungicide-amended agar) and in vivo (foliar-sprayed tomato plants and dip-treated grapes). In both tests the benzimidazoles, benomyl and thiabendazole, and the dicarboximides, iprodione and vinclozolin, were used as reference fungicides. In all experiments benomyl and tebuconazole proved to be the most active fungicides. The transfer ratio, which is defined as the ratio between the EC50 (the concentration inhibiting growth by 50%) of a particular fungicide determined in vivo and in vitro, was lowest for benomyl. The transfer ratio of tebuconazole was comparable to or lower than that of vinclozolin. Hence, no obvious correlation between in vitro and in vivo activity was observed. Field rates of tebuconazole recommended for control of B. cinerea are relatively low compared to those of benzimidazoles and dicarboximides. Tomato leaf homogenates and various biological compounds antagonized the activity of triazoles and dicarboximides but did not affect inhibitory potency of benzimidazoles. It is suggested that the factors mentioned account only partly for the limited field performance of triazoles towards B. cinerea.