Transposition of the maize transposable elements Ac , Spm and Mu is correlated with a lack of methylation. For both Ac (Schwartz & Dennis 1986) and Spm (Fedoroff et al. 1988) methylation of a region upstream and downstream of the site of initiation of transcription results in a loss both of mRNA encoding for the transposase and the ability to excise. We have studied the activation of an inactive, methylated derivative of Ac by using a cycle of tissue culture. A passage through tissue culture results in activation of the element and is correlated with demethylation of the 5' end of the transposase gene. Demethylation and activation may occur as a multistep process. We propose a model in which binding of the Ac transposase to the 5' region of the Ac element prevents methylation following replication. Consequently, demethylation and reactivation occurs.