The addition of 2,4-dichlorophenoxyacetic acid (2, 4-D) to Vitis sp. cell cultures significantly enhanced the production of quercetin 3,7,4 ' -tri-O-glucoside, 3,7-di-O-glucoside and 3,4-di-O-glucoside from quercetin. This enhancement of glucosylation by 2,4-D was also observed in cell cultures of other plant species. The activity of UDP-glucose: flavonol glucosyltransferase (UFGT) in cell-free extracts of Vitis sp. cell cultures increased approximately 10-fold, 48 h after the addition of 2,4-D to the culture medium. The UFGT activity increased linearly up to 15 h and showed a maximal response to the addition of 10-50 mg/l of 2,4-D at 48 li. The promotive effect of 2,4-D was inhibited by cycloheximide suggesting that de novo protein synthesis was involved in this phenomenon. Interestingly, similar promotive effects on the UFGT activity were observed for other phytohormones such as kinetin and several anti-auxins.