Transcripts of various sizes hybridize to the transposable element Ac of Zea mays in most maize lines. A 3.5-kb mRNA with an abundance of 1-3 x 10 of the poly(A) RNA, however, is found exclusively in those lines that carry an active Ac. Plants with two Ac elements contain slightly more 3.5-kb Ac transcript than those with only one Ac. Overlapping cDNA clones spanning most of the message have been isolated and sequenced. The 5'-end of the transcript was determined by Northern hybridization and S1 mapping. It starts at several sites over a distance of nearly 100 bases, contains an AUG-free leader 600-700 nucleotides long, has a long open reading frame encoding 807 amino acids and an untranslated 3'-sequence of 239 nucleotides. Four introns with a combined length of 654 bases are removed from the primary transcript. Radiosequencing of in vitro translation products shows that translation of the long open reading frame begins at the first AUG, even though it is located in an unfavourable sequence context. The transcript is found in all organs investigated, provided an active Ac is present in the stock.