Two-dimensional gel electrophoresis was used to examine differences between the polypeptide patterns of an untransformed periwinkle callus line and a transformed line carrying the cytokinin biosynthesis gene isopentenyl transferase (ipt) under control of a light-inducible promoter. Both lines were cultured for three weeks on an auxin-free medium with or without exogenously-added zeatin, in continuous light or in complete darkness. Firstly, it was found that exogenous cytokinin treatment increased the amount of at least 24 polypeptides and decreased the amount of three polypeptides in the untransformed line. Secondly, a marked decrease in the number and the amount of the polypeptides was observed in the 2D-gels from the transgenic line. Traces of two cytokinin up-regulated polypeptides, the amounts of which have been previously found to be correlated with the accumulation of indole alkaloids in periwinkle cells in vitro were present in this line. Lastly, exogenous cytokinin treatment had very little effect on the polypeptide pattern of the transgenic line. These data show that endogenously-produced cytokinin does not mimic the effect of exogenously-applied cytokinin on the polypeptide accumulation in periwinkle callus cultures, and that the ipt-transgenic line has become insensitive to exogenous cytokinin treatment.