The Ac element of maize has been modified by deletion of 537 bases (delta NaeAc) from the untranslated leader of the transposase gene. In a second modification the cauliflower mosaic virus 35S promoter has been inserted into the truncated leader of delta NaeAc, 21 bases upstream of the natural translation start. The activity of these modified elements has been compared with that of the unmodified element in transgenic flax. Deletion of sequences in the untranslated leader only marginally increased transposition in callus while insertion of the 35S promoter enhanced transposition frequency 7-8-fold. Increased transposition correlated with increased transcription of the transposase gene. The presence of a 35S promoter upstream of the transposase gene, but outside the Ac element, also enhanced both transcription and transposition.